"How much does he eat?"
Translation:वह कितना खाता है?
When "how much" is used together with verbs, such as in this case (how much does he EAT), कितना is always used. Neither कितने nor कितनी is used in those cases.
If however "how much" is used with a noun, all three variations (कितना, कितने and कितनी) can be used, depending on the object.
"how many bananas does he eat" = वह कितने केले खाता है
"how many books" = कितनी किताबें
"how tall is she" = वह कितनी लंबी है
"how tall is he" = वह कितना लंबा है
It's not kitne and the reason is that the subject (the BOY we're taking about) is singular
In English,According to the Subject-verb concord rule, if the subject is singular then the object part will also be counted in singular. Same case with hindi translation too
Hope it helps you
You are correct, both can be used to refer to third person subjects: