I know you are going to get a lot of dog whisperer advice on this one...
but the problem is not new. HE'S A DOBERMAN PINSCHER!

Lovable dogs and beautiful, but extremely terrirorial. I know... I have been chased my many in my day. LOL! My ol' neighborhood was one of punks and hoods (today's gangstas), and they just loved their DPs. Always let them wander loose around their cribs (home's to you non gangsta type peeps) and you could not get within 200 feet of the house before being attacked. NOW I KNOW THAT EVERYONE IS GONNA SAY HOW LOVABLE AND FRIENDLY THEY ARE but...

If they were so inheritantly friendly, why do they make such good watch and guard dogs!!!??? When I wanted a small dog that I could take with me into the fields and walk while he/she hunted - I got myself a Beagle. Because it was inherently likely he/she would do this. If I had a warehouse to protect, a Beagle would likely lick any intruders and wag his tail while they emptied the place out.

Of course, there will be some things you can do to minimize this aggression - like make sure that all and any of your friends/family/associates who are coming to your house on an "expected" visit, bring with them your Doby's favorite treat. And the first thing they do when he greets them is to offer the treat. This way he will begin to associate visitors with "treats" YEAH... Yup.. YUP... "I am sooo happy you are here"!!!! LOL! This will also reinforce that it is Ok to greet people in a friendly manner (while you are present) and not turn him into the "Beagle" that greets everyone with a wag - whether you are there or not - whether they have treats or a gun!!!

But this will all be a very slow, gradual transition in behavior, with a lot of work and reassuring on your part - "as it is not natural" to his disposition. I know you are going to get many, many others who are going to say "something "bad" must have happened to trigger this switch, but I say no - he just got older and is maturing into the dog he was meant to be!!!