This is a very young relationship and I highly suspect that at some point it will cease, but right now it is real and it is your DS's life, a big part of his life.
It will potentially be difficult having this bf to stay, and there would have to be an agreement about certain behaviours, and if he breached this he would have to go. But, I think you should do it. Eyes wide open, knowing it's a massively kind thing to do, and that you will definitely wish he wasn't there and be happier when he's gone.
I would do it with these conditions:
Absolutely no weed to be smoked in or outside, and if you even smell it, he's out, even if he is unwell. You have to be prepared to enforce this one. Not just because of the younger children but also because of the bf's behaviour after smoking it.
If he wants to do that, he has to go out for a walk and you don't want to smell it in the house.
This has to be a non-negotiable.
Then I would say he has to be nice and not cause trouble, and what food does he like, so you can have a chance of that going smoothly.
Many many on here would not do this, but they are young kids, I probably would, just with rules and misgivings, but I'd do it.
Oh, also, timeframe: there MUST be an end date.
I'd say he can stay max. 3 weeks.