Maybe he's just not sure about the whole sippy cup thing - maybe he gets too much from it and can't swallow it? Or perhaps, being a toddler, he just likes the feel of the liquid pouring down his chin...
I'd just keep trying until he got 'er figured out. Try giving it to him times when you know he'll be more likely to chug-a-lug - such as just before you feed him or after playing real hard?
Maybe your mom can give him his liquids from a straw if that's what works?