Just went through this with my gelding, Scooby. Only to the right, and only at the trot was 3 out of 5 lame. (for the details I wrote it all down in my "countdown to APHA worlds member journal - its in the last few pages)...
Anyway, we had the vet out two days after initial lameness noticed. Had him blocked and had an 80 percent improvement - hoof testers showed problem in hoof (vet says majority of front lameness problems are within the hoof). Did x-rays and saw no cause of lameness - diagnosed soft tissue injury but unknown exactly what type.
Put him on stall rest for three weeks, with two periods of 5 days of 2 grams of bute per day to minimize damage caused by inflammation. He also had no heat or swelling.
After three weeks (this morning actually) vet and farrier came out and Scooby was 80 percent better, only slightly noticeably off at the trot ( 1 out of 5 on scale). Did some corrective shoeing (gave him a wedge rail shoe) and he improved to 90 percent - not noticeable.
My vet bill was 500. For lameness exams, nerve blocks, xrays, bute, and updated shots for both of my horses. Bottom line is immediate intervention is always cheaper in the long run, Scooby was sore because he is conformationally challenged and we weren't trimming and shoeing him correctly - correcting that quickly gave his soft tissue (tendons, ligaments, etc) some relief before serious and possibly irreversable damage was done.
Scooby has been given the clearance to resume riding.
Good luck with your guy, I recommend getting a good vet and farrier involved as soon as possible.
There is no passion to be found playing small - in settling for a life that is less than the one you are capable of living. - Nelson Mandela