Glad to hear the young one is booked in for neutering, Tom. It is a very minor op, mine were all absolutely fine immediately they got out of the cat carrier, looking for food and racing around. You should probably keep him in the following day, but no more than that ( your vet will advise).
Who is feeding your missing cat, Tom? Whoever it is, ask them to stop. Then he may well remember he has a nice home where he gets fed! That plus the altered scent profile once the young cat is neutered (as I mentioned previously) could hopefully encourage him to return.
Trying to catch him by running after him to nab him is unlikely to succeed, he will run and he's quicker than you (or your son) and it will probably just scare him.