Vet check, vaccinations if needed, deworming if needed. The vet can also set you on a good feeding program.
He should have a place to eat where the older horse cannot gain access...a solid board across a stall door or corner of a stall or shed. A creep feeder situation.
Don't turn the little guy out with the older horse right off the bat. A stall next to each other or a smaller pen for the little guy for a few days will be good.
Good luck and we will be waiting for photos!
If you ever find yourself in a fair fight, it's because your tactics suck. ~ Marine 1SGT J. Reifinger