It's geting OLD

[pope1982]

Take some evidence pictures, and let him know.

Make it known you know his boy means a lot to you since he has gone to the trouble of training with this costly device, so you thought would let him know he may need to adjust the shock setting or start training again...
(I believe some people use a dummy collar after some time) You'd also like the same done for you if it was your fur kid.

If he says "no way" say "Way.." and show him the pictures.

Let him know how you feel about his dog, or just dogs in general so it's not something you could go without saying or that you are just a picky, nosey neighbor.
Then ask if the dog is intact, with his response go from there with explaining neutering makes them less likely to wander, less likely to cancer, less likely to be struck by a car or abducted (people do this with dogs that aren't EVEN pure bred anymore for cash) less likely to produce litters that may or may not get the proper care and homes....

Good luck, let us know how it turns out please