We use Avent dummies for Jesse and he has no problems with them. The teat is slightly flat rather than being completely round and they seem to sit in his mouth better. We've tried a few different brands and the others never really stayed in his mouth or he didnt like them.
Some babies just dont take to dummies at all. My sister's kids didnt and i inherited all of her dummies. So if he doesnt like them its a normal thing too.