My 13 yro daughter has uspa. My son - 12- has had chronic achilles tendonitis since 7, but it could have been mechanical.
But, then.. weird toes and then some stiff backs in the morning time in the last 6 months. If I didn't have my daughter diagnosed, I would have never ever had a second thought. Both of my kids are incrediably active athletes..sons playing hockey, tennis and baseball right now so who knows the real causes of all the aches and pains.
E-mailed not the peds, but the daughter's peds Rhumy with the very non-descript, relatively under control symptoms. He wanted to see him immediately. Found seveal tendonitis areas, but not real inflammation.
I think the key is to see a Rhumy, a peds specialist, given your family history. They will get a better baseline to guide his development. My kid's Rhumy wasn't willing to diagnose anything, but he took labs and measured for future comparison.
Just a thought.