Please do not administer pred without consulting with a vet! It's contraindicated in juvenile dogs, and it should never be administered without your vet being involved -- even when used as an anti-inflammatory in adults, there's a step-down process to get them off it, and you have to be very cautious about side effects (including infections). You don't know what's wrong with the leg or paw, so don't guess on meds.
Since it's been going on 3 days, it's time for a vet visit. Maybe you can get him in tomorrow, before the weekend?
Off the top of my head, your pup could have an injury (incl. paw pad, toe fracture, soft tissue or ligament...huge range of possibilities), elbow dysplasia (a joint deformity), or panosteitis (growing pains in the long bone which can be very painful and often need pain meds). Or something else entirely. The first step will be a careful exam (possibly including a careful check of the paw pad, doing some joint range-of-motion, and squeezing the long bone for tenderness). If there's nothing obvious that can be detected in the exam, you may need to do some x-rays.
Given his age, I'd ask the vet to look carefully for panosteitis on the imaging -- your dog is right in the pocket of the age when it tends to show up -- but there are lots of other possibilities, and you really need a vet to figure it out for you.
Last edited by Magwart; 07-04-2019 at 12:35 PM.