You didn't mention whether an MRI had been done or whether you had done any functional analysis...i.e. does his tension & postural alignment change when doing activities that bring on the discomfort (ergonomics in all positions & during activities, not just clinically based ROM testing) From age 16 -> 21 there's a lot of "growing" going on, both physically & emotionally. Sometimes you add an emotional element & everything changes.
What has he been doing/tried for the pain (i.e. oral or topical meds, moist heat &/or ice...etc?)? It sounds like the locus is @ the cervical end of the musculature. I find active stretching w/resistance sends stronger signals to opposing muscles to release & Theraband Academy has a routine/regimen for "chronic neck pain" on-line. As long as you register to receive their newletters as a qualified professional, they are free for the printing, & you could positionally adapt them to add more of a stretch where it's needed. Also, you didn't mention if Ultrasound had been tried. I find it focuses nicely on the tight, constricted tendons for more effective release.
Just some tid-bits to think about...Good Luck!