By now it's been about 24 hours...if he isn't back to normal (or close to it), you've done "wait and see" and haven't seen improvement so it's time for the vet. And I mean emergency vet, not wait till tomorrow.
Really no idea what this could be, vomiting and lethargy can be symptoms for so many things. You don't mention his age which would be a factor...young cats, think poisoning or blockage. Older cats...may be one of the issues that affected older cats like kidney failure, diabetes, etc. Really no way to predict, but all need quick attention before it really gets to crisis mode.
Maggie (15 years old) had a similar episode back in December, turned out she had hyperthyroid.
Really no idea what this could be, vomiting and lethargy can be symptoms for so many things. You don't mention his age which would be a factor...young cats, think poisoning or blockage. Older cats...may be one of the issues that affected older cats like kidney failure, diabetes, etc. Really no way to predict, but all need quick attention before it really gets to crisis mode.
Maggie (15 years old) had a similar episode back in December, turned out she had hyperthyroid.