Are you and the former best friend (the person he quarreled with) also good friends, or chummy or connected somehow so that trying to be friends with them both would be like having one foot in two warring camps? If not, then if I were you I wouldn't let this friendship go without more of an effort.

You say he's important to you and your son. Let him know that. Tell him this will be a terrible loss for you both (for all three of you, really) if he continues with this attitude.

If he's uncomfortable coming to your building -- because he doesn't want to risk running into this other person -- make arrangements to go to him. Meet at other locations -- a park, a restaurant, a place where you can all go for a walk, on a boat ride -- or a merry-go-round.

I know it's shallow and silly (and terribly sad) that he thinks to avoid his former friend, he must also avoid you ... but show him how much he means to you. Tell him. Write him. Go see him.

Try not to hold it against him that he can't handle his emotions better. Unless you have an ongoing relationship with the person he quarreled with, you can arrange a relationship with your friend that doesn't involve this other person. It may be tricky and a little uncomfortable at first, but if you are both willing to make it work, you can.

Of course, any relationship takes two ... but if he's as important to you and your son as you say ... I'd keep making the first move -- over and over until he told me straight out to piss off and leave him alone.

My feeling is ... he needs you more than you need him (honestly) -- and if you care for him, try not to let him hurt himself more than has already happened. You may have to be the "grown up" here for a little while.

(Any chance he's dealing with unrecognized depression, the onset of dementia, etc? To have broken off a "best friend" relationship over something trivial is odd behavior. His treatment of you is even more strange. This suggests to me he may not be thinking clearly for "other reasons." Please at least consider that.)