Two days later, he was mildly lame in the right front when trotting on the sand arena footing. My trainer thought it might be a minor abscess. We gave him a couple days off, and our farrier came out. Farrier trimmed his feet, looked closely for any signs of an abscess but didn't see anything. After he had been trimmed, I walked and trotted him over gravel while the farrier watched. He was sound! We turned him loose in the arena and watched him move: still sound!
The next day, our trainer took him out and longed him a little prior to getting on him. He was still sound and looking good. Trainer got on him and did some walk/trot for 15 minutes, during which time we noticed that he might be ever so slightly lame again at the trot, but it was difficult to tell for sure because he is still so unbalanced and uncertain at the trot. I asked the trainer to put him on the longe a little more after the ride just to see where he was at, and he looked mildly lame again.
We have decided to give him a full week or two off and reassess after that -- but I have to say that I'm all kinds of worried and paranoid. Do hoof abscesses really show up like this sometimes? In my experience the horse is usually dead lame for a day or two and then is fine within a few days after it ruptures. Is it possible that he's still just a little too immature to be under saddle at 3yrs old and this is his way of telling us? I will happily give him all fall/winter off and restart in the spring as a 4yr old if it means that he is a sound riding horse for years to follow.
If he is still looking lame after some time off, we plan to have the vet out... but I'm not sure what he will be able to tell us.