Nevermind the potty seat even though your child is to big for it. My son, who is now six, started out on the potty and then stopped using it and went back to pull-ups until he was four. I then got him to use the regular toilet that he was deathly afraid of so maybe it's good that you can't use a potty. I suggest you get a ring that fits into your toilet seat. You can get it at any Toys R Us, Target, Wal-Mart, etc.. You'll find that he'll be comfortable when it comes time to poop on the big potty better using this. My six year old is still afraid and is still using the ring. I feel like I traumatized him during the potty training and am paying for it now. I suggest also putting cheerios or other types of cereal in the toilet, not a lot maybe three or four and play a game of having him try to sink them by peeing on them. Don't have him start peeing sitting down. Teach by standing and if he isn't tall enough use a stool. This worked for me it was just the pooping and I'm still having problem! I hope this helps! L.