What a cute name for a pony!! Chances are he'll need a pony sized halter; go to your local tack store and just as for a pony sized halter, they should have some in stock. Some ponies' heads are bigger than others though, so make sure you can return it if it doesn't fit!
First off, make sure he remembers what a halter means - it means that once it's on and you're around him, it's working time. Then you can see if he ties properly and get him used to being touched all over again - brush him, make sure he can pick up all 4 feet to be cleaned, rub him all over, around his ears and nose.
Then I'd see if he lunges, and just work on getting him back in shape. Lots of walking to start out with, minimal trotting, then over time you can start making him work up into a trot (this is the gait that's best to build endurance at!) and introduce a bit of cantering.
... I could go on, but I'm sure you'll have your hands full with just that for now!
I'm so excited for you Brandon! Can't wait to see pictures and get updates on how little Rambo is coming along!
Best of luck,