my son is 13.5 and also playing competitional tennis - currently he is playing the mantis power poly in the mains at 21kg with the mantis power syn in the crosses at 20kg.
i think a 17gauge = 1.22-1.25mm poly in the mains would be o.k., both the babolat pro hurrican tour and the tecnifibre black code would be interesting. i'd also look out at the pro's pro blackout, which in my opinion is a a(almost) perfect replica of the black code at 1/3 of the price.
a syn gut in the crosses is a feasible approach, but i would take much care in regard to the gauge - a 16g (1.30mm) syngut would stiffen up the stringbed too much in my opinion, i'd rather go with a 17g. according to my experience, the thinning out of the syngut and the notching of the poly mains would be occuring at very close time intervalls.
matching a multi from this point of view is much more difficult, and i must also confess that the xcel has been beyond my price point - roughly 20euros per pack is simply too much for a cross in my opinion.