sippy cup

anyone have problems with sippy cups and bottle nipples caving in??

my sons 14 1/2 months only uses the bottle for bed

but everytime he drinks a bottle he sucks the nipple in so i have to pull it out.. (he uses size 3 dr brown i went to size 4 but they are to fast for him)

even with his sippy cups! hes still getting use to the hard spout cups any idea what I can do ?? ftm