I'd ask him about it too. He's your friend, so if there is a problem he should have the guts to tell you face to face.
I had a friend that I knew since 7th grade. When we were teens, he had a huge crush on me, but I didn't feel the same way. In his late teens, he started dating one of our mutual friends. Well, at the time I thought she was my friend.
In university they announced they were getting married, and I was happy for them. I even saw her at school one day and gave her a hug and said how great I thought it was. And then I waited for an invitation. All my other friends were invited, and since I was one of P's oldest friends, and was also friends with K, I just assumed I'd get one. But it never came.
A week before the wedding, my mom was at a church function and saw one of my other friends there. He had been invited to the wedding and asked if I was going. My mom replied "No, because she never got an invitation. He said, "Oh I'm sure she's invited! Why wouldn't she be? The invitation must have gotten lost."
I never did get an answer as to why I wasn't invited, when every single one of our other friends were. I can only guess that it's because she was still jealous of me and the fact that her soon to be hubby used to have a crush on me. Oh well. I lost a great friend because of her.