well 125 beats/min is above the range of normal in this age group..
and every degree rise in temperature causes an increase of 10 beats/min
upper normal limit of this age group is 110 beats/min and if his temperature rises to 100F (normal 98.6F) then still his beats should be 120/min and not 125..
so consider getting his "TSH" checked if he has some of the following symptoms and signs aswell..

1- sweating on the palm or feet
2- warmth on the palms or feet
3- increased stool frequency
4-if he is loosing weight
5-if he feels hot even in winters
6-if he remains irritable and cant go to sleep easily
7-if he has tremors in the hands when he spreads his hands and separates his fingers
8-if he has increase of apetite

check if there is any swelling infront of the neck