Even though he's larger, he's still just a youngster so any saddle you have is going to be far too big for him. What I do when I've got one that is too young/small for a real saddle is I'll get them started using a bareback pad. Mine has a cinch on it where I can tighten it up enough to get them used to the pressure but there is no actual weight on their back and no rigid tree or anything that could possibly pinch them or hit a pressure point anywhere.
As for what to do, you can do nearly anything with one that young that you can do with an older one...except ride. I've known of lots of folks that will substitute ground driving for riding with their young horses. Lots of folks ground drive their colts all over the place. You can also pony him on trails if you have another horse to ride while you lead him.
Other than that, just work on everything that he'll need to be a good horse later on; leading, giving all parts of his body, make sure he respects your space at all times, ensure he'll stand perfectly for the farrier, teach him to stand tied, give him baths, touch and rub and squeeze and poke all over him (comes in handy if he ever gets hurt and you have to doctor a hard-to-reach spot), expose him to scary objects, take him for hand walks around town, etc. Like JDI said, though, keep the sessions short and make sure to give him plenty of time to just go be a horse.
I have a friend who takes his horses jogging with him the way most people take their dogs LOL.
This is my latest yearling (forgive the improvised cinch, she is about half the size of the last yearling I used it on LOL). This is what I use to get them used to something on their back and around their belly without using an actual saddle that would sit down on their withers or pinch their shoulders.