Have you tried poly/poly hybrids? I’d go with both soft copolys to start with...shaped in mains, smooth/round in crosses. This will facilitate maximum snapback action in the mains providing easy access to spin. Signum Pro poly plasma and yonex poly tour pro are great cross strings. I rarely break crosses so I go with 18g SPPP but 17g yonex PTP also works well. As for the mains, youd probably want to go with a shaped string for spin/bite but it may take some experimentation. Shaped strings provide higher launch angle, round provides lower launch angle, jus depends on preference but I usually dump a lot of balls into the net with round poly as mains. I saw someone mention hyperG but personally, I’m not a fan cuz it doesn’t provide easy access to power. Signum pro X-perience is a better version of hyperG, hexagonal shape provides more sharp edges for increased bite vs square shape of hyperG, not to mention it is softer for added power/comfort. If you are truly concerned about the health of your son’s arm, as long as he uses correct technique and stretches out the muscles/joint/ligaments regularly, he should be fine. Depending on the conditions of where he plays, adjust tension accordingly.
Hotter weather -> higher tension
colder weather -> lower tension
I have about 4-5 racquets that I rotate and I try to use the softest setup that also gives me adequate control to keep the ball in. SPPP and yonex PTP are GREAT cross that I am for sure certain should be used in the hybrids. As for possible mains, the list could go on and on but Ill jus name a few other than X-perience that might pique your interest...
Dunlop Black widow (nice but shortlived lifespan...2-3 weeks max?)
ytex quadro twist (havent personally tried but read many great things about soft/spin)
genesis typhoon (same as quadro)
isospeed cream (very nice copoly that feels soft and dareisay creamy? but not shaped)
diadem solstice power (amazing spin, star shaped string)
cyclone tour (twisted poly similar to quadro twist...i HAVE tried this but felt like the twist aspect of the string provided some unpredictable responses at times)
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