We had the same concern our first trip to Disney......these are some things for you to consider:
How will he feel if you try taking him on a height restricted ride and the "worst" happens, he's denied by either the entrance or the secondary CM? If he has the personality to be able to just be sad and move on, then I'd book him the thrill ride FP and hope for the best. If his personality is one that he would have a meltdown, then it may not be worth the emotional roller coaster (no pun intended, but it is a good pun, lol). I always talked to my girls and reminded them often that they may not be able to ride because the CMs wanted to be sure they were safe and secure and belted into the ride....so when we were denied it was a "oh, darn" instead of a temper tantrum (which sadly I've seen many times with parents who tried to game it). I think it really depends on the child's personality and possibly whether the parents made him understand the possiblity that he can't ride.
If he can't ride with Dad/Grands, what would you do while you wait for them? You mentioned you can't ride those particular rides, would you be able to ride "tame" rides? Would the things you'd be riding with him be things where the standby line isn't normally very long (i.e. not something like Peter Pan or 7DMT). If you'd be wanting to ride something "tame" but that would be best with a FP, then I'd book that so he has something to ride. If you'd be instead using that time to just wander the gift shop or go to a M&G, you won't really "need" the FP so book something you hope he can get.
I'm guessing based solely on the photo and join date in your original post, that this is not the first visit to WDW. So, with the assumption that you'll be going again when he's well within the height requirement, I'd also say to remember that if he can't ride them SAFELY this trip, there is always the next trip where he can ride them safely. It's not as dramatic as "if he's too short he will die", but it could be his head won't rest in the right place and may get banged up, or he won't be tall enough to see over the car, or whatever. I know how easy it is to get caught up in that "it's only 1/2 inch, he's not going to die"....but remember that Disney doesn't make these restrictions to be mean and upset "height challenged" children....but because the experts who created the ride have deemed it unsafe for a shorter person to ride safely.....sometimes it is obvious why, but sometimes it has to do the physics and aerodynamics and other nerdy science stuff, and if that little difference in height means he won't be as safe as possible, is it worth it? As his parent, only you can answer that question. Since we knew that first trip wasn't our last, I was happy to wait until it was as safe as possible for them.