Just hop on and ride!! No but seriously, don't be too put off by people saying how insane they can be. Some you really can do this with. But since you are only doing groundwork now, here's some suggestions....
I don't know why Puck says to never let them back up. In fact, backing up in hand I believe is VERY important for an OTTB to learn. Because most OTTBs don't know how to back up, and it's not their natural instinct. Everything they have ever known is FORWARD FORWARD FORWARD. So you really need to work on STOP-BACK-SLOW. Teach him to back and whoa on voice and hand motions. Teach him to longe, NOT running around in circles longing, but teach him how to walk and whoa first, then trot, then very far down the road, to canter. Most OTTB's don't understand leg signals, but they are also smart and sensitive, so they will usually pick up on them quick. Practice moving him over with your hand or point of a brush.
You can also work on teaching him to give to contact. OTTBs are taught to lean into contact. You can put a soft bit in his mouth and put side reins on him, very loose at first, then gradually tighten them (although don't ever make them "tight). You're not trying to put him in a frame and crank his head down, so don't even worry about it. Let him figure it out for himself, even if it means him running around with his nose in the air for awhile. You can also practice just holding one rein and pulling his head to the side and holding it until he gives, then release immediately (dont pull him tight, just enough for him to want to "give"). This will be huge in later training.
When you get on, I HIGHLY suggest having somebody there holding the horse, because OTTB's aren't trained to stand still for mounting, most riders are put on while the horse is still walking. This will be good low key practice as well. Practice mounting and dismounting without going anywhere until he is good and quiet with standing still. You might have to take baby steps with this, as you don't want to just force him to stand. He might be confused at first. Which leads to the biggest point about OTTBs...
As several people mentioned, OTTB first reaction is to either run or blow up. They don't handle pressure well and can get frustrated quickly. Take everything in baby steps, lots of praise. Never make them feel "trapped". If they are starting to get really upset, sometimes it's ok to just let them go forward. That's their happy place. Go forward, move away from whatever is frustrating them, and come back to it later.
Just go through the basics, acting like he's an unbroke colt that doesn't know anything. You will find most things he knows just fine, and you will find the holes that need to be filled in. All 4 of my personal OTTBs were pretty much jump on and go ride. And they did awesome, but down the road those little "holes" that were missed showed up when you start to do any real training. Don't be intimidated, most OTTBs are not nearly as difficult to retrain as people think, especially one that's already had his downtime. Just be aware he might have a little extra spunk now that he's getting some good nutrition. Sounds like he will be a good boy.
And lots of trail rides