How low? And do you mean Total T3, or Free T3? Can you post his thyroid panel with ranges?

I have low Free T3, and yes, it is something to be concerned about. It suggests hypothyroidism, though it may not be caused by the thyroid gland itself. Depending upon the other numbers, possibly your son is not converting T4 to T3 well, for some reason. You need to provide more details to ascertain what might cause this. It can be caused by autoimmune disease, medications, adrenal issues, pituatary issues, low iodine, etc., etc. -- you need to provide more details.

Free T3 largely regulate metabolism, so if it is low,a person may gain weight (or be unable to lose it even with diet), feel tired, have a low body temperature, and feel sluggish. Yes, it is cause for concern.