Okay when people engage in sexual activities that gives out signals like "I like you", "I'm attracted to you", so that is how he perceived it. When you said you aren't into him romantically, but you were willing to get sexual he probably got the gist that sex stuff is an OK for you so he suggested what he did.
You should know that men develop feelings with girls they get intimate with and especially at that young age. These first experiences will shape how you guys see the world, other people and relationships in the future. If you wanted only these sexual stuff but nothing else you should have told him earlier, after that happened for the first time. You could have presented the situation as you did here "Hey I really enjoyed what happened between us here, but I don't wanna have a relationship or anything more than this as it might effectively ruin our friendship, and we are really close and emotionally support each other so I don't ever want to lose you as a friend. We might lose interest into doing what we do now, but I want our friendship to be permanent." this would convey much clearer message than saying "well I'm not into you", cause you made it like you're using him for your sexual gratification, but he is not attractive enough for you. You probably really hurt him and he lost a chunk of respect for you so he said "Will you suck me off then?".
In male/female friendships it is most commonly guys who develop feelings for their girl friend. So in future if you really don't want to be romantic or involved with your good friend who happens to be male, don't lead them on. It is not fair.
So, life lesson here. Not many if any friends with benefits scenarios work out with a happy ending. Almost always, one will develop true feelings for the other. And it ends/ruins the friendship. No, don't continue to 'experiment' sexually with him. In fact, that's best to do with a partner you care for. Giving yourself away too easily will be something when you are older you will wish you wouldn't have done. Do not keep having sex. And now that he is hurt by you, expect he will let others know you are a willing partner to experimentation and it can ruin your reputation. Seriously, this stuff happens. So, take a break. If you continue down this path, you absolutely need to get on birth control and use condoms regularly for std protection. good luck