However, if he is not suicidal and has a prolonged history of
morphine/opiate abuse, shows mild,if any,
symptoms of intoxification and is otherwise "normal" then he should be fine. Whilst 300 mgs of morphine is considerable for someone who is opiate naive, it is relatively routine for the experienced addict.
Anyways, like i said, based on the information you provided, i am left to fill the cavernous blanks with speculative assumptions and wild guesses.