Utterance: I'll be coming [back] later than him.
1) Than is a comparative adjective.
For example: This ball is bigger than that ball.
2) If you compare two people using pronouns, the standard grammar is:
I'll be coming back later than he will.
subject pronouns are used for comparisons: more than, less than, later than, earlier than, etc.
3) In everyday speech, it is now acceptable to use an indirect object pronoun (that is: me, him, them, me, us [her and you don't change] instead of a subject pronoun.
The standard usage is:
I'll be coming back **later than he will****.
In the standard usage the subject pronoun is used and the auxiliary verb or the verb be is used also. With be, you repeat the verb.
A) The girls are running faster than the boys are. [standard]
The girls are running faster than them. [informal]
The girls are running faster than they are. [standard]
B) They speak better Chinese than us. [informal]
They speak better Chinese than we do. [standard]
c) His sister is taller than me. [informal]
His sister is taller than I am. [standard]
Depending on the circumstances, one might want to watch one's ps and qs here.