Calling the narcotics unit is spot on. Make sure you have as many details as you can get. That will help them and they will take you more serious. Get things like vehicle descriptions with plate number, physical descriptions of everybody that lives in the house, the times cars are usually pulling up to the house with their plate number if possible. The more info the better.
They will then pass that info on to a street level unit that can hopefully start to buy from him and eventually get a warrant and rid you of the problem. This is not an overnight process and you will need to be patient. Going this route the neighbor will not be able to point to you as a snitch, it could have been any person he sold to that got pinched.
Many departments have emails set up for cases just like this. When you talk to the narc unit all for a email address or a good way to continue to pass info to them. They are not going to be able to watch the house all day and you might see things they miss. Write them down and once a week send them the info. You never know what little detail you get might be the key to taking down a drug ring bigger than just that house.
I would not recommend you deal with them in any way. If the guy really is selling even if it is only weed that does not make him an ok guy. I have seen plenty of people assaulted, shot, and stabbed over the dumbest amount of dope. Stay away from them and see what the police can do.