Will not poo on the potty!!

My son is 2 and 3 months and is dry in the day (and most nights), always asks to go to the potty for a wee and rarely has accidents, he has done brilliantly! But.... he gets really really upset at needing a poo and holds it in screaming and crying help and wont go on the potty. Today he got so upset that he asked for a nappy on and went for his afternoon sleep early. He was very regular when in nappies and never made any fuss about it, not he hold it in and then every other day we have a stressed out morning of screaming and crying and sitting on potty, getting off etc. he gets a special treat when he does have a poo on the potty (he's only done 2) and i really don't know what else to do! HELP!:\?