i really like this guy at my college, i saw him once out of school by chance and we had a really good conversation.

It's positive that you had a good conversation, but this says nothing about whether he fancies you. I can have a good conversation with a dozen women a day, but don't fancy all of them. When people who fancy each other first meet they tend not to have good conversations, but have very brief conversations, because one or the other person is nervous. Typically the amount of conversation you have builds up each time you meet, if after several times of meeting each other you are having amazing conversations, he will sit up and take notice and may start thinking of you as a relationship prospect.

Since then hes been friendly but not as chatty.

I really can't say, you haven't given me enough to go on and it sounds as though you have only just got to know this guy.

Do you think its because there are other people around? Ive liked him for ages now but because he has a girlfriend i havent said anything.

If he has got a girlfriend, then yes it could be because other people are around. If a man doesn't want to lose his girlfriend then he won't risk being seen talking to women too much. If he doesn't value his girlfriend or his girlfriend trusts him a lot, then he won't mind and will talk to any women. If he knows his woman will not find out you are having a conversation, then he will talk freely, but this is only when you are both alone, without any prying eyes to sprag him up.

What should i do?

He's got a girlfriend and you don't know whether he is happy with this girlfriend. You will have to presume that he is happy with his girlfriend until he tells you otherwise. For that reason, I only see you making a fool of yourself by making any kind of move on him. My recommendation is to see him as a friend and if you find out his girlfriend is not going well, then you may be able to step in.