I agree with MVPlaya. Almost nothing gets a guy happy and excited when his girl asks him "what do you want me to do?" that in its self will make him happy. and if you are willing there are a few things that neither of you probabaly think of:
1. stroke his prostate with your finger. This means either rubbing the spot between his balls and anus. or actually sticking your finger up his ass and touching his prostate through there. You really need to persuade him if you are going to do this because alot of guys will not let anyone near that area.
2. Stroke his balls while you go down on him. Personally i wouldn't like this but heck, he might.
3. use your hand while you go up and down don't just use your mouth stroke him will you go down.
Love is never fair. Someone will get hurt whether you know it or not.