Cheating? Maybe but not necessarily. Is he under any stress? How's his career? Any drug or alcohol use?
The rest of this will sound cold but someone has to say it.
He may also just be getting "bored" with the relationship.
Trying to please him, long term, is not a good idea. You're going to get frustrated, as you are, for accommodating his needs and not taking care of yourself FIRST.
People can and do grow apart. He's a boyfriend, not your husband. While two years seems may seem like an eternity it really isn't. Moving on might be an option for you. Yes, it can be difficult as you are likely very comfortable with him and change is always scary. There is someone out there with the same needs as you.
really 9months is a long time. and now he starting with not giving it up when you want it , only when he wants it. thats a big No NO ..its sad to say that you have had arguments about it and nothing has changed. it could be that he is cheating but also it could mean a lot of other things.
hows his health ?
does he have a stressful life style ?
but look at the bright side ya two are very young and if he is cheating dont worry there are other out there dont lose hope