I would call your Endo.  If it were my son, I’d probably just go ahead and give a shot of Solu-Cortef just  be  sure, since it’s difficult to determine whether a child this young is becoming lethargic or just being a normal sleepy baby. 

This is just my opinion as to what I’d do with my own kiddo.  You best bet though, is to call your child’s doctor.

Hope things go well for you.

Blessings,