Why is my boyfriend so distant lately?

For the first month and a half that we were dating my boyfriend was awesome-he was really sweet and funny and hung out with me every chance he could. He would 'test boundaries' to see how far I would go with him, but it wasn't very far because I haven' been ready to do much.

Over the past week thought he's completely changed. He's stopped trying to do anything with me, which includes kissing me or even holding my hand. He's also seemed kind of distant and quiet when I'm with him.

Like today, we went to a water park for my birthday with 3 of my friends. He was playing games on his phone half the car ride up there and when I sat to close to him he seemed to pull away.

He sat by himself while we were in the wave pool and almost didn't come on the rides with us because "5 is an odd number. Someone will have to go by themself. Go with your other friend, you invited her." I just looked at him and said "i invited you first, and I want to go on rides with you." He didn't seem very sure that I actually wanted to hang out with him.The rest of the day he was just out of it and quiet.

I've kind of been thinking that he's frustrated that I'm not ready to fool around much, and that's why he's so distant, but that doesn't explain a lot of his other behavior. Any ideas on why he's changed suddenly?