He's likely too young to be having erectile disfunction due to any health problems. I seriously doubt weed would do this at his age either. I'd be willing to wager its stress, he is moved out of his mom's house, probably feeling uncertain and unsafe as far as 'where to go from here' etc and that can take away from being able to focus on feeling good sexually.
There is absolutely nothing wrong with you, he loves you enough to leave with you and you have always been able to get his fire kicking before - its the surroundings, its the change of circumstances in his life that are likely causing this. Bringing it to his attention, freaking out about it.. isn't going to help.
The more things settle down the more it will improve I am sure. It's likely your mind has wondered to worries sometimes when having sex.. or thinking about something that makes you lose focus on your orgasm or arrosal but with women it goes unnoticed unfortunately for guys its pretty obvious.