Newly adopted lab mix is overly protective

I just adopted a dog ("Bubba")from a no-kill shelter, where he has lived for most of his life. He lived outside, in a chain link kennel about 10 by 10 ft - there were a *lot* of other dogs and cats, and he got along with all of them, but he hasn't been around a lot of people so he was a little shy when I met him. When he was 4 monthes old, his owners had to give him up because he was getting too big. He's now 1 1/2 yrs old and 80 lbs - his mother was a yellow lab and we think his father must have been a catahoula cur. He's a bit larger than I was looking for, but he's sweet and actually very calm and laid back most of the time (I live in an apartment), and he's still playful when I want him to be.

He's become very attached to me of course in the last 4 days since I took him home with me, and actually isn't displaying bad separation anxiety when I leave the apartment - he wants to go with me, but hasn't torn anything up, or pee'd or pooped anywhere while I was gone - it's almost like he's been housetrained.

He's met a lot of people at my apartment complex and my parents as well, and actually spent all day outdoors with me and my parents on the first full day I had him (Saturday). He's been fine with all of them, until yesterday, he started growling at my dad for no apparent reason. I've seen him growl and bark at other people, but he always stops whenever they get up close (then he wags his tail and pulls his ears back). Why would he suddenly be so overly protective of me? We had to keep him outside in my parent's back yard all day yesterday, but I was with him 90% of the time - I was working on my truck. Of course, whenever I went inside, he would howl and bark to get inside (it happened a lot yesterday). I've heard the way to handle that is just to ignore it, and eventually he will stop. My parents have an older american eskimo (male) and he's still scared of the new larger dog, so we've been keeping Bubba outside most of the time when we're at my parent's house. It got so bad last night that Bubba was jumping up on the back door and barking (aggressively) at my mother through the window in the back door! Of course, my parents are both starting to get scared of him now (they're in their 70's).

What can I do to stop this behavior? He also tried to attack another dog at my apartment complex today (I had him on a leash) for no apparent reason. The other dog was a smaller shepherd mix and was acting a little shy - maybe he sensed it? I really need to get his growling and aggressive barking under control, especially at my parents! Thanks very much for any advice or input at all...

Hi "Bubba"-master!

First of all i want to say how much i admire people who help pets with a bad/uncertain/problematic/tragic history. It is so important to take care for them!!!
I understand that it gets problematic when your parents start to worry about Bubba. But you have to see that he has been with you and your surrounding just for couple of days. He must get used to his new family and i can imagine that he is a bit confused about the new situation, so many people around, another dog, you who took him out of the kennel, sudddenly inside the house where he cannot follow you ....
What i find very important is to show him that you and your parents are the ones who take care, who deal with people, other dogs, "problematic" situations from the very beginning. Start working with Bubba on a daily basis. Work on commands like "come", "sit", "stay" aso and praise him whenever he did well, give him treats immediately after he did well. But make also sure that he cannot follow you everywhere you go, he is not your bodyguard and he must understand and accept that. Also when you are at home a lot, let's say on a weekend, do not let him follow you all day long. Make him stay somewhere, give him something to play with, etc.
Make also sure that he can drain his energy when you walk him. Give him the chance to run, to feel his power and to get a bit tired afterwards, that is very important!
Walking him on the leash is also an important point for your relationship. Whenever you walk him, take him on the leash and have him walk by your side. He should not walk in front of you or anywhere behind you. You decide where to go, how fast you walk, and he must be by your side. By training this he gets the security that you are his master (in the best sense of the word!) and that he can trust you and must not care for "human" things.
Whenever you sense fear in him, ignore it and get him out of the situation as calmly as possible. Whenever you get excited, Bubba will as well.
One more thing concerning this training: if your parents could do that as well, it would be good, because in this way Bubba can accept them as his masters as well.
So, i think that's it for the moment. If you have any further questions don't hesitate asking, i would be happy if i could help.
In the meantime: all the best for you, your parents and Bubba.

Thanks for your quick reply. I'm actually going to see a dog behavior expert today to see about the possibility of training him (and myself) out of the bad behavior. The rescue organization is willing to pay for it ($350!), so I can't complain, but it may take a while, and where I live there are people and other dogs everywhere, so I'm not sure if I'm willing to go through with it - he has tried to attack a couple of people since my first post, one time in front of my apartment manager! One or two more times with people in my apartment complex and they'll kick me or him out. The behaviorist says he's just trying to scare people off, because now that he has bonded with me he doesn't want anyone else to get between us (he's not actually trying to hurt anyone, just scare them off). Thanks again for the advice

Hi, thank you for your answer.
I think it is very good to have a specialist watch you and your dog interacting with each other. And it is great that the rescue organisation is paying for that!!!
I do not wish to interrupt any therapy that might have already been started, i just wanted to add that if you train your dog, start doing it at home, where he feels save and where he has no distraction. Train him walking by your side at home, put him on the leash and walk him inside, have him stay by your side and praise him, give him treats whenever he is doing well. He will surely understand quite soon what you want him to do.
So, now i withdraw because i do not want to disturb.
All the best for you and your Bubba.
Michael