Hello, I am sorry to hear about Ralphie's behavior.

I think it is an excellent idea to fence in your garden as that will prevent the problem from occuring.

I don't blame your neighbors for being fearful of a growling, snapping dog.

Ralphie is hitting the age of social maturity. This happens later than puberty and is a combination of physical maturity and the age at which a dog figures out where he stands in the social pecking order. When he figures out what he can or cannot do in a social group. A dog his size goes through this at about 18 to 24 months. He is pushing the limits and has found that he can roam and while he's having fun exploring he doesn't have to go home if he growls and snaps. He may also be somewhat intimidated by your neighbor's unhappy body language and likely being scolded. Especially as he know he really shouldn't be where he is.

A fearful dog is more likely to become aggressive. He may be fine with this neighbor in other, less intimidating circumtances, especially with you there as you give him the confidence that nothing will happen to him. He may also be fine when people come to your home as he is confident and comfortable there. Their body language is likely much different then the neighbor's when they are trying to catch him.

So the best thing is to not allow him to get into this situation in the first place. He is a fellow that needs you there to boost his confidence and to be in comfortable surroundings.

To help him learn to get along with people in different circumstances perhaps a refresher obedience class is in order. This will get him exposed to lots of people and pets outside the home and reinforce his confidence that if he behaves he will be fine and doesn't need to react aggressively.

If he is not neutered I highly recommend that you have him neutered. It will decrease his desire to roam and find a mate and as testosterone is an aggression hormone taking that away will decrease aggression as well. Neutering also prevents prostatic disease in older dogs as well as the incidence of perianal adenomas and hernias.

Best of luck with your boy. He is young and now is the time to reshape behaviors that aren't appropriate.