S.F.
as an adult with OCD, i admire your family. I wish anyone would have recognized my problem when i was a child. I am happy to see that you are supporting your son and helping him through this. The other suggestions are great, i would have recomeneded the same (minus the medication). I will also suggest two books that i found after I noticed my daughter (who is 8) struggling the way i was when i was child. She has not been diagnosed, but i see it.
1.Talking Back to OCD: The Program That Helps Kids and Teens Say "No Way" -- and Parents Say "Way to Go"
by John S. March
and (a bit younger but still nice)
2.What to Do When Your Brain Gets Stuck: A Kid's Guide to Overcoming OCD
by Dawn Huebner
Of course there are more, but these were my favorites.
Best of luck to you and your family.