nah, he randomly spams it when he's set to defensive and aggressive.
The only way to be sure he only uses it on you would be to make sure all the other mercs were out of his range at all times, otherwise he's reaching for the old stimulant.
My advice is don't upgrade him with the 32 power, he only gets stim and a low damage, high aggro grenade.
Personally, I am traps so force field generator is my mez protection. My medic doesnt even get the first upgrade, is set to passive follow until I switch him to defensive follow when he begins healing. He also has perma grant invis on him too 
is there a way to assign a macro to make the medic only cast stimulant on me? (so it wud be perma mes protection)