Would hoof boots help with this?

How big is the indoor? Smaller size might make cornering difficult. Do other people who use the arena have the same problem?
Does he slip if you lunge him?
What about if you do ground work with him?
Does he slip if he is turned loose in the indoor? (when there are no other horses in there of course).

No, hoof boots will make the problem worse, unless you use the classic easyboot and have them drilled for studs.

Your farrier isn’t being honest with you. Yes, sometimes different shoes will help - those that are beveled the entire outside of the shoe, and have a decent channel in the center. What kind (mfr and type) does your horse wear currently?

Also, you can take a tip from foxhunters and have your horse’s back shoes drilled and tapped at the heel for small studs. You screw in the studs when you want to work, and remove them when the horse is turned out. The empty hole is packed with a rubber plug for easy removal. Studs are actually the best thing for preventing slippage as they provide some really good “grab” on almost all surfaces.

5chestnuts, He is currently wearing the best shoe for the situation which is beveled all around. It’s the eventer shoes. I thought about tapping for studs in back, but this footing is weird. It’s all sand, so nothing to grab onto. It displaces very easily, so if he really dug in he would just hit the hard bottom which I believe is either concrete or crushed gravel. In another shoeing he will have the borium tips on there anyway, so I’ll see if that does anything.

phoebetrainer, yes I do think size is a factor. The arena is not tiny, but is only wide enough to do a 20 meter circle. I’m thinking it’s most likely 60 x 20 meters. Funny you should ask about the other horses. The other TBs in the barn also slip. I think ultimately it’s just a combo of footing, walls and size.

Luckily my horse is super smart about footing and will just not pick up a gait that he thinks isn’t right for the situation. I’ve been giving him more days off between rides and quitting the rides before he can become tired. Today he was downright silly and did some nice W/T/C before tiring out about 30 minutes in at which point we called it a day. I was thinking about adding Previcox over the winter to ward off any inflammation, but am on the fence about it.

It sounds like your horse has made it clear what he thinks of the footing. It doesn’t work for him. Since you already had an injury potentially related to the footing, I would be hesitant to work in it. Deep footing puts a greater load on the muscles, increasing injury there. Tired horses are much more prone to tendon and ligament injuries, just like any athlete that overworks the muscle to lose form.

If you’re committed to staying at this barn, you could try a couple weeks of just walking to condition him. Start at 15 minutes and work your way up to an hour over a month. I know you say he is well-muscled for what you’re doing, but you might change your perspective to thinking about conditioning him for deep footing. You can test him at the trot and canter after a few weeks to see if that helps. Think about this like coming back from an injury. You need to build him up for his work there.

Or, change barns to a place with better footing. Here are some things to read:

http://equimed.com/health-centers/lameness/articles/the-dangers-of-riding-your-horse-in-deep-sand
http://www.thehorse.com/articles/12792/get-a-handle-on-your-footing
http://www.chronofhorse.com/forum/showthread.php?344830-consequences-of-riding-in-too-deep-footing
http://www.chronofhorse.com/forum/archive/index.php/t-322800.html