I saw my neighbor share a joint with his 14 year old son.

I can only give you my personal opinion, although my being a social worker does play into that.

First of all, I would not call the police.  If you call them, the result might be that your neighbor is charged for his own marijuana use.  Is that a result you could live with?

I can understand your concern about an adult sharing marijuana with a 14-year old.  But let's back up a bit.  If you observed your neighbor offering his son a beer, what would your reaction be?  Providing that to someone who is underage is also illegal.  But families and parenting styles differ.  Whether we like it or not, some parents do allow their kids to drink limited amounts of alcohol now and then.  Would you call the police, if it was a beer?  Without getting into a discussion of the differences between alcohol and marijuana, and their potential to lead to addiction, the underlying issues are much the same.

If I was still concerned about having what I saw, I think I might call the local child protection agency, and ask ... without giving the names of the neighbors involved ... at what point would they become concerned enough to intervene?  At this point, all you have is the one incidence that you observed, and you are not able to report a pattern of this kind of behavior.  They might give you suggestions about what else to watch for, in terms of repetition and frequency, and also what other concerns about this man's parenting might be relevant.  My guess is that responses from such an agency might vary widely, depending on things like location, community attitudes, and even the personal opinions about marijuana of the person you talk to.  But most would still respond professionally to your concerns about the son's age.

I understand the dilemma you are facing.  You want to do the right thing.  There are many people who would say that it is none of your business.  And there may be many others who think you should intervene.  Rather than reporting it, how about having a conversation about it with your neighbor?  Say that you saw him sharing a joint with his son, and that you were concerned about this.  Say you even wondered if you should report him to police or child protection.  And see what he says in response.  I think that I would want more information about the neighbor's parenting style and attitudes, specifically about alcohol or drugs, before I would report anything.  But as I said at the beginning, that is just a personal opinion.  In the long run, you will probably do what you think is right, based on your own values about parenting, and about a citizen's responsibility to report things that concern them.  I merely suggest you take more time, and get more information, before deciding what to do.  Your concern is laudable.  What to do about it is more complex.  Good luck sorting that out and deciding what to do.