It's kind of hit and miss with grown males. Some will take to other males just fine, and some just won't. Neutering may help but I guess in your case, it's not an option. Best to do intros gradually in a neutral environment, just like you would with dogs. I will be starting intros between my new boys and grown male this week. I'll start by putting a new male with my grown male in a bathtub (neutral, no marked areas, easy to keep them in the same area but big enough for either to run away). I will only do this once or twice a day for short periods until I'm sure they'll do fine together. Then when I am ready to put them in the same cage, I will be sure to completely sanitize (hydrogen peroxide & vinegar) the cage, wash all beddings and rearrange all hammocks and toys so it's not familiar territory for anyone.

It's not recommended that a rat be kept alone, no matter how much time and attention you can give it. Even 3 hours of one-on-one time doesn't even compare to the constant companionship another rat provides. However, there's only so much you can do if a male just won't accept another rat. A spayed female would be a nice alternative if you're willing to spay one girl. Male and females tend to accept each other a little more easily.

I should mention that a 10 gallon is really way too small for a male.. I feel guilty just putting them in a 10 gallon for a couple minutes while I clean my cage.. nevermind permanent housing..