Very young but inadequate sex life with BF

My BF and I have been dating for over a year. We are very serious yada yada. 
I'm a very sexual person and ever since we started to date our sex doesn't last long. No matter how many times in the week we do it, he cums within 2-5 mins. Rarely it lasts 15-20 and if it does its cuz he's cummed 2-3 times already. He doesn't want to talk about it or get anything checked out because he's only 24. 
AND! As of late
I feel like I can never catch him in a horny mood. He likes to smoke pot a lot and he says pot makes him not horny so I better get it in before he smokes or the rest of the night is toast. 
I don't know if it's stress, or the weed, or me because just tonight he says I make sex too much of a priority. 
Can anyone give me any helpful advice!? 
And please don't make this about our communication. 

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