Try lunging him on a line for about 15 mins. All of my Arabians have a lot of GO and they understand that they can buck and fart on the line, but once they settle in and get to work, then they come off the line and I mount up and off we go. If they act like they want to bow up once I'm on, I hop right back off and they go back on the lunge for a few more minutes to get their brain back. I put them through their paces, walk, trot, canter, reverse, back, whoa and see how they do. My stallion especially, needs that time to play up a bit then settle in to working. If I'm patient and give him the time, we have an awesome ride. If not, well, he's only 4, so sometimes we have an 'interesting' ride that isn't quite as smooth as I might have preferred!
Best thing I've found for trying out these horses with lots of GO is to get them focussed on me, then when I get on, I let them have as much head as I can give them. If they want to go out at the trot, that's fine, but then we trot until I say we don't trot anymore, and that's always several minutes longer than THEY want to continue. That way I'm not always having to hang on their faces or fight them.
I always take a really deep seated saddle for a trial ride, so that way if they spook, spin, buck or run, I'm good to stick. Now I expect if this boy likes being ridden, and he's used to it, he might be fresh, but I bet he'll be a piece of cake. I can pretty much toss any of my Arabs out into the pasture for 6 months and then go grab one and go for a ride without any trouble, that's just the way the breed seems to be. They are more alert and forward than a lot of breeds, but they were bred to be that way. A dead head in war would get you killed, a sharp attentive horse would probably save your life.
Oh, another thing my Arabians do that no other horses I've ever owned have done: When they spook, they sort of startle real hard and then freeze. I call it the elevator drop spook. They snort, jump and spread their front feet a bit, so the front end drops a little lower and then they freeze solid until they've give the situation a real good look. It's always lasted long enough for me to say, "OOOH OK, it's just................", settle in my seat and collect the reins and let them go on without a fuss. That's the real key to my guys, I settle and say, "OOOOH that's all right, not a big deal." and they listen to my seat, my voice and my legs, if I don't get all tight and worried, they don't either.
I'm claustrophobic so I won't ride in a round pen on a bet. If you're comfortable riding in a 60 ft round pen to start, go for it, but I much prefer to get on and hit the trail, where I feel like we have some room.