Hi
I expect/hope you've taken him to your vet by now. There's a variety of potential problems for your cat's lethargy that are unrelated to the neutering, especially as that was a while ago now. Anaemia isn't very common in a young cat but can occur and has a variety of causes, some are infectious and could be caught from other cats. Infections would be high on my list to check for why your cat is lethargic anyway but anaemia is another possible cause of lethargy. If it is present it is serious, so I hope he's been checked, had some tests and is being treated now.
Cathy Wickenden BVet Med MRCVS
Barton Lodge Veterinary Centre
1 Midland Road, Hemel Hempstead, Hertfordshire, HP2 5BH
Tel: 01442 216048 24 hours
[
www.bartonlodgevets.co.uk]