There are many reasons why your horse might be lame, but problems in the legs should always be taken seriously. A lame horse is unable or unwilling to place weight on one hoof, and is usually identified by a hobbling or tentative gait.
Read more: How to Care for a Lame Horse | eHow.com http://www.ehow.com/how_2314768_care-lam
find out what leg he is lame on. Think of what possibly could have caused the lameness, saddle fit, past few days work, horses age, etc... Exam all the horses legs thoroughly for any swelling, heat, or anything weird looking and cold hose all 4 legs. Check the horses hooves, see if there are any weird color (red) on, in, around the horses hoove. it could be a bruise. Next put the horse in a dry covered area. In the stall, or covered paddock. preferably sand. something with soft ground. Give the horse bute. call the vet and make an appt. Leave the horse in the paddock or stall overnight.
Vet comes, gives diagnosis, follow vets rules, listen. do ultrasounds if needed, do xrays if needed. do whatever is needed, then follow up with the recovery process.