With the yamalink should I get a heavier rear spring, in your advertisement it says 10% but is that only for over 170lbs? I want to lower it as much as possible and have 4 inches of race sag.
Our reply: Steven, Hello. My 13 year old nephew is the same height, weight and ability as your son. He recently started riding a YZ125 while still riding his Super Mini and 85. At first he put on the 1.5 with a lighter spring, but eventually went to the stock spring and the 1.5 YamaLink. Today he is on the YZ RACE.
My recommendation for 99% of lightweight YZ 125 riders is to consider a lighter spring, but for the 1% of kids like your son and my nephew I say try the stock spring first (the too-heavy one) and reset sag and go in clockwise on the high-speed compression (the big knob atop the shock, not the flat blade) in 1/3 turn increments until the bike handles big g-outs and flat landings.
My 16 year nephew weighs 140 and races local pro, borrowing (stealing) his little brother's YZ for Schoolboy when his own KX450 is tore apart. He ran the stock spring and the 1.5 for a few months, and loved it but said he'd probably go to a slightly heavier spring. But since the bike has the RACE YamaLink on they have zero issues....and they're not just saying that cuz I'm their uncle. Dang kids!
Hope this helps. Let me know if you have more questions.