Yeah, tell him you don't really like to give BJ's.
Seriously, while making something enjoyable for your partner is cool, I think it's BS that he isn't willing to basically roll over and let you have the reigns for your own pleasure for a bit.
If you are in a relationship, you are going to have sex possibly hundreds or thousands of times. You don't lose anything by letting someone else do what they want to do sometimes(assuming that thing isn't something you find activly repugnent, or immoral or something). Sex should be give and take. If you think about it, you are making the same sacrifice when you do less favorite positions instead of being on top. There is no reason he can't do the same for you.
To me this sounds like more of a case of a selfish boyfriend than your lack of skill.
Try to move your body in the same direction the penis is pointed, rather than trying to bend it. That may be his problem or the fact that you are in charge..
Another neat way is in a strong, armless chair with you straddled facing him on his semi-reclined lap. You might want to pad the chair. Tell him you just want to try it just once. Betha he'll do it again.
Get his penis erect. With his tailbone area on the chair, his penis will be fully extended and appear really big and long. You should comment on that as you lube it up, then sit easy and slowly on it. Start off slow with short strokes. Have a cloth nearby to wipe off sweat and maybe some water to drink. You both will have your hands, mouth, lips free to stimulate and your imagination is the limit.. Take long strokes as you can without him popping out. Done right, you will orgasm many times. He will love it.
Giddiup.