I agree with the advice below - your son is old enough to have the sippy cup introduced into his daily routine, but he is still very young and it is age appropriate for him not to want to use it. Let him chew on it and carry it around with him (with water in it only).
Offer it to him at every meal and snack - as long as you give it to him consistantly at mealtimes, as he gets older and his understanding grows he will associate the cup with mealtime and drink out of it just fine. Don't expect him to use it at this point - he is still a baby, afterall. My daughter is now 22 months old, and I starting introducing the sippy cup at about 7 months - fortunately for me, she took right to it, though she did prefer to chew on the nipple instead of really drinking from it. Also, most children the age of your son already "know" how to drink and suck from a straw without any help from you, so invest in a sippy cup with one of those soft straws that you can tuck inside the cup when not in us. He might take to that better than the regular old cups. My daughter likes them both equally.
Dont worry about it - you dont need to "train" him at it, but let him learn on his own!