K.U.
I would let him do what he wants to do, and if it ends up being too much -he's stressed out or his grades start to slip - then you will need to let him know something will need to be dropped. If you tell him no, he may just end up angry and resentful that you don't think he can handle it - if he really can't, let him find out for himself (but don't tell him "I told you so." - just be sympathetic and supportive).
I wish my stepsons were that ambitious and motivated - they always ended up quitting every activity they started, their grades are so-so and now in 10th and 12th grade, you still can't get them to do anything! The younger one will be 17 this August and he still can't be bothered to sign up for driver's ed! And the older one graduates this year, never applied to the college he was interested in (or any others) and still has no idea what his plans are for fall!