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Do you know if he has urinated in the last 12-24 hours?
Try pressing on his abdomen over his bladder- do you feel anything hard or does he act painful?
The reason I ask, is because urinary blockages are very common in male cats. A male cat that is acutely lethargic, vomiting and acting painful would be a urinary obstruction until proven otherwise. This is a life threatening condition, so I would probably try to get him seen as soon as you can. If there is an ER nearby, I would utilize them.
Here is a nice write up on the condition and treatment:
There are other things that it could be- a fever, pain elsewhere, an infection, ect. No matter what, the falling over and extreme lethargy make this an emergency situation where I would not wait to get him looked at.
Let me know if you have any other questions.