Hi, Tracy suggested sitting LO on your knee facing outwards and helping him drink hand over hand to give him the idea. Also only put about an oz in the sippy cup as it is quite difficult to pick up if it is full.
I would offer a bottle of water if you are concerned about dehydration at all, it may well be he is getting enough fluids without large quantities of water. You could try modelling the sippy cup, I drank some tea out of a sippy a few times and my DS saw it in action, it motivated him to have more interest. You could also try a straw, my LO liked that although it did mean I had to hold the cup so it is not quite so easy to let him get on with it (you may need to model the straw too, you can also dip the end in water and put your finger over the end to trap some in, then release the small amount in LO's mouth to give the idea).
hth and maybe other have some ideas too.