Most horses are more or less full size and developed at four or five. I would guess that you're looking at more than a year before it would be worth trying the saddle on him again to see if it might be a fit. And even then, it will depend on the build of the horse (shoulder angle, back length) and the build of the saddle (gullet, bar angle, etc). That saddle would probably never fit my QH, since he's got a short back, but would look gorgeous and likely fit two paints I know pretty well.
Have patience. A two year old is pretty young to be doing any serious carrying of weight anyway, so if you're just looking for him to carry an empty saddle around as part of training, fit is less important. If you're starting to back him though, you'll need to try several saddles to see what fits, and be ready to have to change out or switch saddles every few weeks since he's still changing and growing so much. Just be careful with your youngster. (And your new saddle... I love the tooling!)