Myself and couple of other fellows use 16G Ashaway Kevlar x 17G Ashaway ZX Pro. The best of our lot is a 5.0+ 27 yr old veterinarian who played DIII tennis. I have tried the other brands of aramid or kevlar string and none work like ashaway. If I were you I would get one of his extra rackets and string it 52-54lb 16G Ashaway Kevlar in the mains and 54-56 Zyex Pro in the crosses. I have tried many other combos with kevlar including gut, multis, polys, and syn gut. I've tried Zyex in the mains and Kevlar in the crosses, no go. I have tried the extreme tension spreads and they didn't do anything for me. My hitting buddies and I like the pairing around 60lbs in the mains, 62 in the crosses. But your son make like an even deader kevlar. The magic only appears when the Kevlar is paired with Zyex, when Kevlar is in the mains, in my opinion.