I don't want to ruin my friendship, but I need to tell him I like him.

You have to decide for yourself if telling him is worth risking your friendship. I was in the same situation not so long ago, and for me the right thing to do was be honest. That doesn't necessarily mean it will be for you, though.

This is the sort of thing you have to take case by case.

 

In all honesty, I think you should just get it out there. These kind of things can escalate very quickly and end very abruptly. In other words: very bad for you.

Tell him you like him, maybe even love him and that you are aware of his sexuality and that you appreciate his friendship and his kindness and favour towards you. Hear what he has to say and decide FROM there.

Usually HEARING your crush affirm or disregard you gives one a clear idea of the person's true feelings towards you, how much they value you and where you stand presently, or futuristicly.
It can also bring some sort of closure or puts you on the path to closure as KNOWING is always better than ignorance. Some things in life are worth the bliss of mystery, like...God, and the wind...Lady Gaga's true hair colour, etc.
But situations like this are detrimental.

The sooner it's over, the better. And you can move on as friends, lovers or otherwise.

That's just my 2 sense.

Goodluck

 

If you don't tell him now and later on decide to tell him, he might take it more harshly that you kept that big of a secret from him for so long. If you tell him, say it was for his sake and he needed to know. He might take it better if you consentrate the conversation towards him.

 

Do you NEED to tell him? If you are certain beyond a shadow of a doubt he's rigidly straight, you have nothing to gain by telling him you want a relationship with him.

You could alternatively try saying something like "If I had the chance to be with you, I'd take it." It's less aggressive and awkward than "I 'like' like you" and reinforces the idea that you are alright with things staying the same if he says no.

EDIT: Reread the original post. I highly doubt telling him will make your feelings go down; if you're anything like me, that will make it much worse. :S