poor Ziggy …. I’d say the same as Joan I’d get another vet opinion.. and also another FeLV test too..its recommend in kittens to test again 2-4 weeks after first test as it take about two weeks for the virus to show in bloods after exposure and that’s why it’s so important too also retest positive FELV test 2-4 weeks later as some FeLV positive reading on the snap test are actually showing the anti bodies from a transient infection ..
Honey is a good trick for a lethargic kitten Manuka honey is even better again but can be expensive.. you could be dealing with a parasite as kittens new to be wormed ever month untill they are at least 6 months old and a single celled bacterial infection like Giardia or coccidia can really make kittens lethargic and all cats that have been in contact with a giardia or coccidia his need to be treated and all litter trays and litter tray areas completely disinfected .. get another vet to test his stool for parasites
and goats milk might help him feel better if he’s an upset stomach