How was the 1 rep max determined? Was it based on a true 1 rep max OR a calculation using max reps (like doing a max triple and calculating a 1rm based on those numbers)?
If it’s the later, try different online calculators/calculation methods. Jim includes a formula in his books as well.
Other than that, all I can say is it’s not the end of the world - your son will eventually hit a plateau (as we all will) and you guys will have to recalculate then, which may end up giving you a better 1rm approximation anyway.