Slow flow sippy cup ?

DS has never made it beyond the level 2 nipples because he would have choking episodes and then refuse to drink.  We have been trying to introduce a sippy cup and he actually took it today, but he was really struggling to swallow because it was coming out so fast so after a minute he pushed it out and wouldn't open his mouth or take his bottle for us.  Does anyone know of a slow flow sippy cup that I could get for him?

TIA!