First of all, is Bo a Basset - I ask because Bassets, male or female, don't generally behave like this although if he was taken off the streets (which I suggest, would be unusual for a Basset - to be found on the streets) he/she may be into overly guarding the property. Secondly you may have to put up some fencing, or some form of barrier between your fencing and next door. It's only been a few weeks however, so as he settles down with you, things may change between him and the dog next door.