If you boost him with a level 110 character, yes, Dungeons.
If you level a character together with him, probably rushing the pre-7.3.5 level 10-20 quest zones and then the zones at least one of you knows really good, as you can finish quests really fast.
You could try and see, if you two are strong enough to push dungeons, though - it can happen that you will get some players without knowledge, which fail and so on.
No, just level up with your bud, like every guide proposes. As RAF is now 50% Bonus exp now (was 200% before 7.3.5) and does not stack with heirlooms (which it did before), you have to level up the normal way.