Thanks for you replies. He does eat as much as usual and his behaviour is the same. I haven't seen him depressed.
If I do notice that he goes down I will take him to the vet. ( I can't afford to take him now)
Try calling your vet and explaining what you've observed about Tomky since the neuter. He may have some suggestions for you or may be willing to do a followup to the neuter for free or a reduced price. You don't want to let an unexplained weight loss go without some kind of vet contact. Here's some be healthy vibes for Tomky.