I havent read the replies so apologise if I repeat anything.
I would say if you think it is sprained he should have 5-7 days rest preferably away from other horses and places where he can run around. Stall or small paddock rest is best.
I can be a little over fussy so a week may be a little excessive but I guess it really depends on the severity of the injury. If however he doesnt seem any better after 5 days or so I would get the vet out to him as he may have done more damage than at first thought