IME you are expecting quite a lot for him to take on both a cup and drink a quantity of milk from it all at once and so young. Perhaps you just need to review quantity and/or frequency of each feed especially if he is just starting on solids. If he is also breastfed I think he would compensate for any lack of formula anyway.
Probably best to persist with one type of cup rather than confuse the issue and one which doesn't require too much suck. DD had the Avent one but didn't really master it until around 6 months. Ds had several ones with 3 holed spout and handles (no suck required.)
hth