He is used to the light and simple power of an Air cooled, single overhead cam, trail bike engine. I would recommend staying on this platform and just get a bigger bike. The YZ125 is a good bike in size but not in power, it will definitely be too much him. A great choice is a 125cc/150cc 4 stroke like a ttr-125 or a crf150”F”. These bikes have the same engine on a bigger bike. Of course they will have more power than the 100cc but they will be much more comfortable to ride. If you want power though, I recommend a Honda crf150 “R/RB” these bikes have a race engine and pretty much are 250cc mx bikes on a smaller bike that would fit him nicely. I recommend a smooth transition. Going from a 100cc 4T to a 125cc 2T isn’t the smartest thing. Xr100 to a crf125/150F is much better. The Honda crf150R is a great bike to transition from a trail bike to a race bike. Remember, you are not just looking for seat height, make sure it’s not too much bike for him. Good luck!