Pay attention to Blackduc's comments. If your son is anything like my 12 y.o. daughter, he'll have an opinion on what bike he would LIKE to have. My daughter was outgrowing her XR80, but she didn't want to move up to our XR100. You see, she is a Chad Reed fan, so she wanted a blue bike! So, in Dec., we sold her 80 and bought a used TTR125 L (large wheel). She now can't wait to go riding! The XR100 is about the same size & is lighter. It's only drawback (stock) is that it has the stiffest clutch pull you will find on any modern bike of any size. I recently ordered XR80 clutch springs for our 100; My 14 yo daughter complains about it and rides our TRX250 4 wheeler instead.
Your son will probably be able to ride anything suggested here, but will WANT to ride more if it is the bike HE wants to ride. The XR 100, TTR125, DRZ125 & (?)Kawasaki 125 are all VERY similar in size & performance and are what I would recommend, especially since you mostly trail ride. I have found it usually costs me $50-250 net, after reselling one of my kids bikes. So, if you shop well, finances become a non-issue.
My daughter's goal is to improve her skills on the 125 so she can earn a YZ85. Good Luck & remember to ride as much as you two can! :cool: :ride: