I agree that if a horse is getting obviously sore for work, that the level of work was either too much, or the level of the horses fitness was too low....now in saying that I don't mean to make you feel like you've done the wrong thing. It happens to a lot of us.....like the unexpected gallop across the field after a long ride to chase some problem coyotes away...for fear that they'll attack another one of your dogs!!! Winded horse, sore horse, winded rider, possibly sore rider!
I agree totally with Spotted in terms of bute and it's side effects and also massaging the soreness out. On that note I'd like to add, gentle walking from the ground to keep that lactic acid moving out of his muscles.
Now, if the horse has truly 'tied up' I'd be offering bute, as this can be very very painful, even crippling for a horse.
In my own personal experience, we only ever gave bute to a horse, if it had genuinely tied up and was known to tie up regularly (certainly not a desired condition, but something that can be a problem for a horse regardless of training or feeding) or had a condition that a vet had deemed necessary that the horse be treated with bute. Bute is a prescription med for a reason, and I'd hate to treat a horse without consultation and have something go awry.....
wish you well
I would avoid using someone else's left over bute, try walking it out of him and for sure try Spotteds method.