hey bro. not to be an asshole, but you might want to prepare for the worst for your cat.
sudden weakness in pet usually means something internal. and even in the event that your vet can identify what it is, there's no guarantee they can handle it as most people don't want to spend much money on an animal's health.
ergo, the vet don't have enough experience.
speaking from experience here as my labrador died before 2 years old from internal cancer(or at least that's what the vet said. and she also told me upfront that the operation will be expensive, and even then there's no guarantee. understandably, my parents at the time don't want to spend the money on the operation. and i'm not at working age yet.)
why do i say this? because while my parents won't pay for the op, they did pay for the "hospital room". and although i visited him everyday, i'm not by his side when he passed. i kinda regret that. so IF (and this is a big if) it seems hopeless, it might be best for him to spend the last of his day with you, instead of inside some cage in the vet's.
but if your boy is getting thinner but still quite energetic, chance are he have some kind of parasite. probably worm.
i hope for the best.