Hey there. We have no way of determining whether or not this is a potential overdose situation based on the information( or lack thereof) you have provided. His considerable weight could be more of a risk factor rather than a protective element considering the respiratory problems many overweight people can encounter.
If you suspect he is stupefied and behaving in a manner which suggests he has over dosed( nodding, slurred speech, lethargy, irratic and/or slowed breathing) then by all means telephone emergency services immediately. 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.
If you google kadian it will give you drug information and included in that info is what to do in the event of an overdose and what things to look for to determine if it is indeed an overdose. Don't mess around here - and call 911 if he doesn't look right. Now is not the time to second guess the situation.
Good luck to you.
Kadian is a Morphine derivative. If he starts acting strange, call 911. That is quite a high dose, I don't care how big he is. Don't take any chances, make the call if you suspect anything.