Quite a few youngsters develop splints, theory suggests they are caused by concussion (trotting/ cantering on hard ground) they are basically bony growth around the splint bone at the back of the cannon bone, usually on the front legs.
Whilst they are forming they can be painful, hence your horses current lameness, but once they have formed they should no longer be sore or cause any further problems.
To a trained eye they will be noticeable so you may be marked down in showing classes but other than that your horse should be able to continue life normally including jumping etc.
