What do you do when the guy you love is dating someone else?

Very tough situation.

Unfortunately, we can only change ourselves, we can't change others or their actions. I don't know this guy so I'm only putting some ideas out. Some people, for whatever reason live very conservative lives, they have a particular routine day to day, they don't venture out beyond their limits. But, any human has an intense desire for adventure, and it manifests itself differently in all of us. It's possible that you're friend, who is in control of everything in his life, wants something that is out of control, to achieve a balance.

In this case, it's his out of control relationship with someone who doesn't want to be in control at all. I know people like this, I work with people like this. A guy I worked with lost his job over a similar situation.

Just remember, you can only change you, you can't change him. He has to want that for himself. The best you can do is be non-judgmental, and be a supportive, caring friend.

Ask him out
If that doesn't work, and he still rejects you, ask me out
haha

you should tell him that you like him or you will never be able to have a chance with him if he does not know that you have feelings for him. GO AFTER HIM!