My son is 13 and his foreskin is fused to the head of his penis what should he do?

Urologist. glanular adhesions of foreskin to the glans penis which can't be reduced manually should be evaluated by a urologist. I typically will do a circumcision ( or revision of curcumcision) under anesthesia as outpatient .
Consult needed. If it is truly fused it would need to be released surgically. The main thing you never want to happen is for it to slip down part way and form a tourniquet that could cut off the blood supply to the far end.That could be catastrophic. See a urological surgeon to discuss your options.