Keep my firearm hidden and offer to join up with him. If he asks what I offer show him my weapon, but don't disclose my ammo count.

You didn't mention that my neighbor is male or female but I have both in my family so - he/she is alone and might be interested in companionship as well. This can help deepen the bond of joining together.

Now - if they just refuse outright - I produce my firearm and take what we need to survive. Then decide whether to leave a potential enemy alive or simply close this chapter and move on knowing they won't be a problem in the future.