H.B.
Your poo issue with your son sounds very commmon. Although I didn't have this issue with my daughter, I know of several moms with boys that would do exactly what your son is doing...so I can only offer 2nd hand advice about what they did! One boy goes to pre-school all day and never goes in his pants (he is pee trained)....as soon as he arrives home he asked for his pull-up and would hide behind his dresser in his room and go. His mom slowly (and sneakily!) would take longer and longer getting the pull-up ready when he would ask and have the urge to go until one time she very happily and excited said 'oh goodness I can't find one..hurry, come into the potty, fast..." In all the chaos and excitement, he went on the potty! He would still ask for it but she would give it in the bathroom and started making him take it off to clean up...which he did not like. The last thing they did was to not buy anymore pull-ups, and he had no choice because he would not do it in his underwear.
Another mom simply did not offer an alternative (no pull-up or diaper to poop in). She made a HUGE deal about him picking out some super cool underwear (in hopes that he would not want to mess them). When he did mess them, she cut them off and threw them away. He only pooped in his supper cool underwear twice and didn't want to ruin them!
Most other moms I know just wait it out....making a fuss over the fact that they just arent comfy or ready isn't necessary and only stresses everyone out, especially the kid. Most Dr's will say that they will do it when they feel right about it, with subtle gentle coaxing :-) If he refuses to go anywhere but outside at your home, then starting school shouldn't be an issue. I would not by anymore pull-ups and have him wear underwear.... but I would do it gradual so he doesn't stress.
p.s. - from what I have experienced and heard from other moms - girls are far easier to potty train, so don't worry, let them lead and you follow...it's so much more natural and smoother.