Hi & welcome,
I took him to the vet yesterday and he said that he has some fluid on his stifle. He said not to ride him for about five days and then I am giving him Previcox for a couple of months. He also suggested giving him a glucosomine supplement to his food.
Fluid on his stifle indicates an injury of some kind, but did they do any tests to see what else is going on? Esp as it's been goiing on a month now, I'd be wanting more diagnostics & answers. Also not riding him for only 5 days but giving previcox(an osteoarthritis drug??) for a couple of months... not sure about this idea, and it's possible it will relieve the pain enough for him to do more than he should for the injury. Also for a long term supp, you might want to look into Turmeric instead of/as well as glucosamine, as it's a good anti inflammatory too.
His other recommendation was to put shoes on him and that this will help him.
It certainly won't help any non descript injuries, and depending on other factors, could make it worse! Peripheral loading of hoof capsules with rim shoes, and concussion on hard ground with metal shoes & peripheral loading is very not helpful. BUT when there are stifle/SI issues and the horse's hooves are deemed too low in the heel, shoes may be proscribed with wedged heels to 'improve angles'. So if for eg he's got 'no heels'(usually means long but crushed flat) behind, if he's steep crouped &/or he's 'sickle hocked', then that could be the reason the vet suggested this. I'd be inclined to use hoof boots with frog support pads, under the frog, not walls, if this were necessary. I'd also be inclined to consult with a chiropractic vet or other knowledgeable bodyworker, and consider xrays of hooves & 'upstairs' too.
If he has a stifle problem - can he recover enough to trail ride?
Yes, probably, but it depends on what the problem & how it's treated of course.
Another question - I was going to send him to training for 2-3 months - how long should I wait after this heals before I send him?
Once it's healed, it's healed. Tho without knowing exactly what's wrong... I would however, inform the trainer of the 'injury', so they can be cautious of it.