Our dd has recently started to pooh on the loo, she had been suffering constipation since xmas (when we potty trained her) we tried everything to get her to pooh on the toilet and occasionally she would do it. We got little toys and wrapped them up and sweets and praise and sitting with books etc we brought a padded toilet seat but tbh i think what's cracked it is the fact she's a bit older now, she understands the pain in her tummy feels better when she pooh's, we let her do it in her nappy but when she did we said good girl for having a pooh but do it in the toilet next time, I started going for a pooh when she was in the bath playing (tmi i know but i was trying everything!)
She's doing it frequently now (over the past week or so) she likes the padded toilet seat I say do a pooh and then you can have some sweeties and watch tv on my bed for 10 mins so when shes had a bath she sits on and i go out of the bathroom and she shouts me when shes done, so far so good! We're giving her so much praise and attention for being such a big girl, I also told her after her birthday if she does poohs on the toilet everyday then she will be able to go to nursery (she really wants to, like her big sister goes to school so this has been a big incentive for her, though a few times shes said you haven't taken me to nursery! but i explain again that she has to have her birthday first etc) She's 2 (will be 3 in sept)
Thing is we took her to the doctors and asked for an appt with a consultant at the hospital so we could get her off the medicine, now we've done it all ourselves which is typical but i'd rather cancel an appt than feel we have no support or back up.
Hope you can not worry too much. He'll do it in his own time. Give him the nappies and eventually he will want to be a big boy and do it on the loo. I do understand how you feel though, when dd2 was suffering from constipation I felt everyone thought I must be such a failure and not feeding her properly, dd1 also suffered for 2 yrs and I thought people were judging me I know I've done all I can though.
Chin up, things do get better try not to worry x