How do I tell him? HELP ME

Me and this dude have been sexually active for a little over a year. We don't have any sort of relationship out of sex the only time we contact eachother is when we want it that can be once a week sometimes two weeks it just depends.  I have been feeling a little funny and think I might be pregnant and I don't know how to tell him. He's a good guy but then again we don't know everything about eachother cause all we've ever done is have sex. I just don't want him not happy or saying it's not his. I go to the obgyn tomorrow to know for sure but I'm wondering should I tell him I might be or how do I do it ... Help please.