I will try to make this as short as possible.
I've been seeing a guy for the past three months and I really like him, but I don't know if he's just using me. We started having sex quite soon, but we always cuddle and chat afterward so I don't feel used. We stay at each others and have a laugh with each others friends.
We've been on dates, he seems to care about me, occasionally holds my hand, talk about pretty personal/deep things together and we really get on. I think I'm falling for him but I don't want to fall any more in case he doesn't really want me and I'm just easy for him.
He said early on that he didn't want a girlfriend, we were practically going out anyway and that he didn't know what I was thinking about it. At the time he said it I didn't worry too much as we both agreed to talk about it after the month's holiday (suggested by me).
However, the last week of term at Uni he told me he had been talking about his girlfriend, I asked who he meant and he said 'you'. Also, during the holidays he said we would have a couple of proper dates.
Were not exclusive and I have maybe sometimes acted defensively in order to hide my true feelings from him, which may make him think that I don't want anything more serious.
Should I ask him if he wants to be exclusive and take it further or should I assume it's just sex and leave now?

I think you need to end the confusion by talking with him.

Give him a chance to express what he is thinking and feeling and decide together if there is more to the relationship. If there isn't, then you have your answer and can make a clean break without wondering if your assumption was wrong.

When you talk to him, don't put words in his mouth. If you aren't certain what he means, ask for clarification. Have him do the same in return.

Three months isn't really long enough to know if there is an everlasting love, but it should be long enough to know whether you both want to work at a longer relationship.

Good luck and no matter what take care of yourself.