What kind of bottle are you using? I only ever used the Playtex Nursers, so my son took FOR EVER!!!!!! to figure out a sippy cup. I first introduced one to him at 7 months; he was about 13 (14?) months when it FINALLY clicked. Like your son, he LOVED using the sippy, AS LONG AS MOM WAS HOLDING IT FOR HIM! He just didn't understand that he had to tip his head to get the milk out. To help him get the idea I used a hard bottle first (Playtex Airvent with the Nurser style nipple) and then switched to the nubby soft tip and gerber soft tip cups. I also found that even before he figured out the hard bottle, he could use a straw cup sipper. I just didn't feel I was getting the straws clean enough, so I threw them away. (We didn't have a dish washer at the time. FINALLY :O) bought one about a month ago. YEAH!)
If you only use nursing style bottles, try a hard bottle and see if that helps him figure it out. I also recommend, as some other moms have, put the stuff he likes to drink in the sippy/ hard bottle, and just water in the bottles you have been using. If he's like my son was, he will not learn unless how he wants it isn't an option.
BTW, I nursed my son to 17 months. I know for sure by 14 months I had stopped using the bottle, but brought it back to wean him off of me. it took about 2 weeks, but then he never wanted me or the bottle again. (At least not until I started babysiting an infant a few months ago. But he knows that their only for the baby, so he doesn't get one.) He's also just given up his sippy cups for anything except water in the last month. (He's now almost 29 months.)
Hope it helped, ~J