No matter who teaches your son, tell your son:
Keep his trumpet horizontal (don’t point it down at the music stand) so the sound has an opportunity to carry and he is not constricting airflow.
Arch his fingers on the keys so he uses his fingertips instead of playing flat-fingered so his fingers are more nimble and he looks more elegant.
Don’t slouch (constricts air).
Don’t tap his foot (to keep time, flex his toes without moving his foot) in order to not distract with movement and noise, and in order to look like he has been trained.
