"Deserves" is correct, but it is only partially because "he" is singular. It is as much because "he" refers to the third person. "I" and "You" are also singular, but they use "deserve." In fact, in the simple present tense, it is ONLY the third person cases that uses "deserves." And that only in the positive form. In the negative form it is also just "deserve"
This a fairly common pattern with regular verd in the English language, Only the third person positive requires an s (or sometimes an os", at the end, Every other simple present uses "deserve" no matter who is speaking, how many people are speaking. Once you can flip that around in your head and realize you only have to look out for one particular case. You should be off to a good start.
So in the affirmative, It is:
1st Person: "I deserve
2nd Person: "You deserve
3rd Person Affirmative: *He/She/It deserves
3rd Person Plural: We deserve
3rd Person Plural: They deserve
And in the negative, notice the only exception is in the same place, third person singular, though it is carried by a different verb
1st Person: I don't deserve
2nd Person: You don't deserve
3rd Person: *He/ She / it doesn't deserve
3rd Person Plural We don't deserve
2 person Plurar: They don't deserve
All regular verbs follow this pattern in the simple present, I believe. So the trick is to just develop keen are for what constitutes a singular third person subject. He, she, and, it are obviously pronoun. The way stare at me all day while I am working actually doesn't bother me is correct, because for all the words in "the way you . . . while I am working" it still boils down the pronoun it.
I hope that doesn't come off too complicated. You can look at the way I use the verb "to use" in that paragraph above and see it follow the same rules. To walk. To work. Etc . . . These are the verbs with the least tricks up their sleeves.