Is it considered boosting if...

That isn't bannable but the community generally looks down upon it

I beleive as long as he is the one playing his own account it is fine.

Sheesh, just get him up to gold at least, no need to go and ruin diamond games cause of the person you're boosting.

If your backpack can hold them, it's ok. Frowned upon and your teammates will hate him once you get into a certain level. There's a reason ranked used to have a max skill difference allotment that didn't allow golds to duo with bronzies. If he is on his own account, there isn't anything against the ToS, just morals.