I am a huge coaster enthusiast and rarely meet a coaster that I don't like.
That being said, I find wooden coasters to be much more jarring than steel ones, and would prefer a big steel coaster with multiple loops and cork screws than a wooden one with a couple of big downhill drops.
IMHO you are correct to be concerned about the head trauma, particularly if he has had concussions previously. He may incur further damage without even knowing it. Even the steel coasters can "rattle" you a bit.
If he were my son I would do exactly what you are doing, and that is WAIT until he's quite a bit older and taller. That's a harsh recommendation, but that's what I would do (and I love coasters with a passion and love when one of my sons rides with me).
If he's ever been seen by a neurologist for the concussion or loss of consciousness you could possibly make a quick appointment there, too, to ask the wisdom of riding coasters at his age and size.
But all that aside, I would do what you're doing. Yes, it's hard. But lifelong neurological damage is even harder. My oldest son is giving up tackle football for that very reason. He's gotten into performing arts and doesn't want to lose his great eye-hand coordination for guitar with one bad shot to the head.
Hugs.
PS: Just saw your location - you must be near Kingda Ka . . . oh I'm jealous. Have been wanting to ride that beast for years! :)