Kids do loose things. Mostly because they don't realize the value of what they have because they really didn't do anything to earn it.
On that note, might I recommend and Enzo Elver or Enzo Trapper blade blank on which the two of you build a knife. Both are scandi ground, less than four inch blades, and make VERY nice knives. You can get them in O1 or D2 steel. You can buy complete kits with the handle scales and pins, or you can just buy the blade blank and the pins and use your own scale material. Decent leather sheaths are available for around $15.
Putting a handle on a blank isn't terribly difficult, but it is time consuming. After your son has put twenty hours into a project that he can proudly call his own, I'm guessing he'll take better care of it than an off-the-shelf knife that's dropped in his lap (so to speak).
Also, these are both excellent knives that will serve him for a lifetime. My Enzo Trapper is my go-to knife for carving, bypassing my F1, Bravo 1, Dozier Elk Hunter, etc..