1st thing: Is he an inside / outside only cat, or goes in & out?
Have to eliminate pests, depending if he's an inside only cat.
Is he bleeding from the bites?
And do you think him constantly scratching the itching bites is causing the hair loss?
If you're sure there aren't any fleas, it could be several things. From mosquitoes to bed bugs to biting flies.
When are you / your partner getting the bites? Only at night in bed?
If so, bed bugs are a possibility. They come out of hiding at night & give nasty bites. If the cat's sleeping on the bed, well...
If you're not getting bitten while asleep, surely you'd notice being bitten?
FLEAS: sometimes they're hard to spot, if not a large outbreak.
If you put on white socks & shorts, then walk in all areas where the cat goes, fleas are stimulated to jump on passing animals. You'll see them on the white sock (or white pants).
Shuffle you're feet along slowly - creates more vibration.
I've used this method many times, to see if fleas were present or had been eradicated - in the house or the yard. It works.