Might be smart to check your son's iodine, T4-hormone, and maybe T3 levels, as iodine affects the thyroid (body's thermostat valve) and T4 is produced by it to be partly converted into T3 elsewhere. T4 and T3 activate heating. If not low, try some tyrosine or iodine anyway.
I have some gut illness with malabsorption, so the above over-the-counter tricks don't help much. I'm also skinny and can't gain weight. If suspecting bad gut, test for food intolerances by mediator release test or immunoglobulin G (expect false positives and negatives), or try an elimination diet (like rice and meat only for a month).
Caffeine increased my muscle tension. Muscle relaxants counteracted it. Now off both, taking lots of magnesium.