Thanks for placing your query on HCM.
Your son's weight is on the 3rd centile of weight chart. This means only two normal boys out of 100 of his age will have weight lower than him.
More significant & worrying would be if he has lost any weight.
TSH is a hormone secreted from brain to stimulate thyroid
gland .High TSH level
simply means his thyroid is underactive and can be treated with thyroxin treatment in appropriate doses.
Normally, underactive thyroid does not cause low bodt weight.
Please consult your pediatrician
who can prescribe thyroxine
& investigate as required to look for causes of failing to gain weight.
Hope you find the information useful.