In Canada, smoking marijuana is legal as long as the baby is not exposed to the smoke from it and as long as the parents are "sober" when caring for their baby.
You can't force anyone to do anything, so all you can do is figure out how to have very clear boundaries about what you will and will not allow in your house, have consequences for people disrespecting your boundaries, and ensure the baby is safe. If you know the parents are using while caring for baby, then make a call to child protection. Otherwise, what can you do? Let the baby's mom worry about him getting a job and if he doesn't, then let him suffer the consequences of potentially broken relationship, court orders regarding child maintenance, etc. Just don't bail him out of the hole he's digging for himself.
I am concerned about the baby and any potential future babies, though. There is research (I had access at my previous job working with families) that shows when a teenage male uses marijuana regularly, it alters how his body produces sperm, which then of course alters the sperm, leading to adverse effects on baby. The same does not hold true for girls because we are born with all our eggs intact. One of our high schools has a health centre in it, and the teens who attend there are given information on this so they can make informed decisions on what to do with their body. Young people everywhere need to have access to this information.