"I am coming with him."
Clearly "onunla geliyorum" is correct, but lesson repeatedly gives an example where word "beraber" is used and accepted as formal form of "with". So, why not "onunla beraber geliyorum"?
no that doesn't work, "ile" is always compact with pronouns
- onlarla (why this is not ONLARINLA is a big mystery)
Not a big deal. The same reason why you wouldn't say bizlerinle, sizlerinle or arabalarınla...
it is still weird, because for benim senin onun bizim and sizin we use the possessive as you can see, we don't use possessive (with instrumental) for anything else.