Wow, lots of good advice on this thread, from many different perspectives. I haven't read thru all of them - there were so many - so I may repeat some points. Bear with me.
I am the mother of five - ages 28,26,24,22 and 17. So I have a bit of experience raising teenagers!
I think it's normal for kids to try out different philosophies and religious concepts - especially if they are intelligent and spiritually or philosophically inclined.
I raised my kids in a very multi-cultural environment, with a lot of emphasis on personal expression, independent thinking, respect for other cultures, and most importantly of all (in my opinion) respect for truth from ANY source. I taught them that though the bible is true - it is not ALL truth. We are a Christian family, but I allowed them to question, study, explore, etc without fear of rejection or ridicule.
That being said, I never hold back my OWN opinion on whatever it is they're delving into - in other words, I kept my apologetics skills honed. But I never - NEVER - ridiculed them for challenging the faith and values they were raised with.
I want them to form their own faith and value system and know it is theirs - OWN IT. Only then can their faith be strong enough to sustain them during difficult times.
But as I said, we are a Christian home and we go to church, don't take God's name in vain, observe Lent and Advent and other Holy Days, and that's the way it is in our house. You live in our house - you respect our faith. You are a minor - you go to church, including Midnight services on Christmas Eve! Funny how the kids always griped and caterwauled about that - but now they are nostalgic about it...
When my oldest daughter was 17, she dated a Muslim guy for awhile. She knew my position on that. But I also knew that irrefutable truth about teenage love - the more you oppose it, the more they embrace it (and each other!). So rather than make some big stinking deal out of it, I let it run it's course, though it was difficult since I personally do not encourage inter-faith marriages. Of course, I didn't expect her to get married at 17 - but you never know...And besides that, I always said "don't date someone you wouldn't marry." And I still think that's good advice!
Anyway - here's my point. I think you should go talk with the imam and learn what you can about your son's version and concept of Islam. Besides going straight to the source for information (always a good idea) this will show the imam that you, as his father, are going to be monitoring this situation. This is called accountability, and the imam surely knows that he is accountable to the parents of the kids in his congregation.
I would continue to learn all I could about the pros and cons of Islam. I'd engage my son in conversations at every chance about it - show him your interest, challenge his knowledge and his commitment, and DON'T ARGUE with him. Apologetics and arguing are not the same thing. Know how and why your faith differs from his new faith. And - show respect for common truths.
Teens are notorious for trying to shock their parents, and for rejecting their parents' ideas. He may be quite dismayed to realize that you agree with him on many of the teachings of Islam - that's UNCOOL!
Oh man, I could have some fun with this one. Learn the disciplines of Islam and then tell your son that you expect him to live his faith. Make him accountable when it comes to the laws and precepts of Islam. But at the same time, he lives in your house. The practice of Christianity is the modus operandum in your house. I would still make him come to church (if that's what you do), and continue to celebrate Christian holidays with the family.
Man, if he can keep all that up, my hat's off to him. That much religious activity ought to keep him out of a lot of trouble!
There's a lot of common ground between Christianity and Islam and if I were you I'd focus on that - you're not compromising your values, or ridiculing his that way.
I have a feeling this foray into Islam will run it's course with him. But if not - there's really nothing you can do about it. He's a young man - he has to make his own way.
Meanwhile, if you see signs of radicalism or violence with him, you must intercede. My experience has been that converts to ANY religious faith are more extreme and often more dedicated than those born into that faith.
By the way, four of my five children are now practicing and faith filled Christians. I have one "heathen" and he still professes Christianity - he's just sort of hypocritical about it right now. But he's young...
Peace be with you and good luck. Keep us posted!