Lethargic is never a good sign and 103 is too high for cutting teeth. There is a "fever virus" going around that lasts for up to 5-7 days with up and down fevers. Things to keep an eye on:
(1) make sure he is still urinating
(2) make sure that the inside of his mough is still moist
(3) make sure there are still tears when he crys
If any of those things stop... go to your doctor or Acute Kids... they are staffed with pediatricians and have after clinic hours, located in McKinney, Frisco, and Plano. If any one of those 3 stop then your little one is dehydrated. Keep your little one dressed down to just a diaper and/or shirt, keep him as cool as you can, don't overdress. Good luck tonight!