Socializing...I need Help!

OK, I'm going to try to make this short and not let myself ramble on.  Of my three dogs, none of them were purchased by me.  They have all been passed down through my and SO's parents.  They were never properly trained.  Buster is probably the best behaved, my dad really worked with him and he listened to him very well, especially off lead.  Max belonged to SO's mother.  She bought him just a few months after their previous dog passed away.  Max received no training, very short walks, and absolutely no socialization.  Max did fine when introduced to Buster and they do fine living together now so all is good at home.

I have been trying to work with him on walks.  I try to walk him when I am pretty sure no one else will be out walking (typically early morning on weekdays) and in an area where most people don't walk their dogs and there aren't any houses to pass that might have dogs outside.  I have begun to notice significant behavior changes in him and he behaves much better on walks. So this week I have started to walk Buster and Max together because I'll be going back to work soon (I'm a substitute teacher) and my time will be more limited. 

I had two really wonderful walks with the two of them.  No problems, no pulling, no bad behavior...it was wonderful and it made me so happy.  I use a harness for Buster, and a halti (and a harness, just in case) for Max (it's the only way I can stop pulling).  Today was a whole different story.  The walk was going wonderfully and we were on our way back home, when a man and his dog appeared not too far ahead of us.  I did the best I could to keep their attention focused elsewhere (which is pretty much impossible with Buster who is deaf, but he did ok, besides his rooing. lol), but Max just pulled and pulled and got the stupid halti in his mouth  and laid down on the ground trying to get it off so he could continue pulling towards the dog.  I was so embarrased.  I just want my boys to behave on walks.  Buster isn't really a problem, he whines a bit, but I have enough control of him to keep walking, but Max is so strong.  I am so afraid of what he may do.  How can I socialize a seven year old dog?  I really wish SO's mother would have done this when she first rescued him. 

I don't think he will do anything to the dog, but you never know.  He always lunges at both people and dogs, and it is so stressful and embarrasing, that I don't even want to walk him.  I just don't know what to do.  The only thing I can think of would be to walk them separately and use treats to get him to focus on me. But would that even work if we see another dog?

Anyway, I've rambled on enough.  Any help would be greatly appreciated.