Obviously a bad situation. Other people may have additional ideas, but here are my thoughts:
1) You can certainly call the NYPD and report the drug dealing. You are able to do this anonymously if you wish. This might get rid of this guy.
2) The kind of house we're talking about and the kind of agreement you have, if any, with your neighbor affects what you can do legally about the maintenance issues. Sometimes, if it's a duplex, you have a specific agreement that was created when the property was originally sold. If it's a typical Brooklyn brownstone or similar, however, you may need to start an action in court for waste. It depends on HOW this is affecting your property.
For example, is the wall between the buildings that is crumbling? If so, it is likely a party wall, and you may be able to sue on that basis. Similarly, if the porches are attached, the area where they meet may be considered an extension of the wall. There may be a party wall agreement on file that speaks to the responsibilities of the parties in this case. If not, you may still be able to file an action for waste regarding the damage he's causing your property. As fair warning, however, this may be an expensive proposition, and if he's not maintaining the building, there may not be a lot of money at the end of the lawsuit.
Finally, regarding all of these issues, you may want to call 311 and complain. This is a good self-help option, depending on how bad the damage to his building is. If it is a danger to other people, the Department of Buildings (DOB) may be able to require that repairs be made immediately. I've seen buildings in Manhattan threatened with this over facade issues.