Whatever her 3yo sister uses!!!
We have the tum tum, tried the munchkin straw one but because it has a valve it was too hard to get water out of
I have a selection of different cups for him and touch wood he has been fine with them. I haven't tried him with water in his bottle as I wanted him to recognise that his bottles are for milk and cups for water. We also use an open cup with a very small amount of water at mealtimes some times x
We use a Tum tum cup and he worked it out really quickly!