V.B.
You may need to have him watch your husband or your older son go poo poo in the potty so that he can see for himself that it's okay (the dolls may not be enough for him to actually get it). I have only potty trained a girl so far since my son is just 7 months old, but she watches me go all the time (mostly because I just never get a minute to myself, but eventually it became a curiosity about potty training). It has been my experience with my daughter that the poop thing is the hardest part of training. She has been in underwear for about 6 months, but we still have relapses in that area periodically. As hard as it is (and I'm being a little hypocritical here since I have a hard time with this myself), try to keep your patience and just keep working with him on it. I wouldn't force him to sit on the potty because that will only scare him more. You could have him go pee pee first and get it all out and then let him sit down on the potty with a toy he likes that is just for the potty.
Letting him watch your hubby or son should help and there are also potty books out there that show boys sitting down to go poo poo and standing up to go pee pee. My daughter has the girl version and she liked it, so maybe try to find one of those.
Good luck! I know how trying this time can be, but he'll get it eventually. At least your son asks for a pullup, my daughter would just go in her underwear all of the time and never tell us. Ugh!