DS gagging and being sick . . .

[neeps1207]

My DS (2.7) has been potty training for just over a week now and its been going ok. He doesn't yet say he actually needs a wee but will tell me he needs to go, usually saying that he doesn't pee in his pants or he wants a sticker. The past few days he has started to get the potty out and go by himself, which I see as real progress and we're slowing stopping the stickers (unless he asks for one). We've only had a few wet accidents, mainly down to overexcitement.
However, he is refusing to poo in his potty. He usually goes in the morning, and we have had a few incidents when out and about, and also at nursery. I always ask him if he needs to poo before we leave the house in the morning and encourage him to sit on the potty for longer, but as soon as he is finished his wee, he is up trying to clean out his potty! What is concerning me is for the past 3 days when he has had a accident in his pants (poo not wee), when I'm cleaning him up he starts to gag and has made himself sick the past few days. I don't know what is causing this and its alarming me. I don't give him in to trouble when he has an accident, just calmly tell him he needs to tell me when he needs to go next time, so I hope he's not anxious. It's also not overly smelly. We have had 2 accidents today, both times he has made himself sick :(
I don't want to put him back in nappies as he is doing so well, but I just don't know how to deal with this. Any helpful suggestions?
Thanks