My "hubby", boyfriend of 8 years, is staying home too. He did terrific with our 3 year old, started to get bored/down about the end of the 2nd year, but it really got bad for him with the addition of a newborn baby boy, now 4.5 months old.
I agree with the moms... playgroups are a good idea. But I think it's even MORE important that he has as much of HIS OWN time as possible. No kids allowed. I work 45 hours a week, and as much as I'm tired, I know it's NO WAY near as much work as he is doing at home. The SAHMs & SAHDs are the real workers, and that needs to be recognized by us out of home workers, both men & woman.
FORCE him to go out. You mention he's antisocial, but does he have any friends? If not, he might need time for online activities, video games, beers at the bar, whatever it is that he used to do before babies came around.
My biggest concern for you is that you are pregnant.... he's going to continue to stay at home with both babies? TWO BABIES ARE WAY MORE THAN TWICE THE WORK. So if it's not working now, it might get even worse with the new (although wonderful) addition. I would set something up now... we had (and continue to struggle sometimes) with my boyfriend staying home with two instead of one. Your boyfriend needs an outlet, and time with you, to feel good about how he's spending his days. The last thing you want is for him to resent the children or you, these can strain a relationship seriously. Good luck! It's a lot of hard work, but we all make it work out... somehow!! :-) K.