Depending on how far you son is from fitting you could look at crank shortners. They also have the advantage of being fast to take off and put back on if you ride with an adult. If this gets you close you can also look at a different set post and or saddle. When my youngest grandson was just a little short I got the thinnest saddle I could find and a straight seat post like you would find on a cheap bike. I flipped the clamp upside down so it was above the saddle rails. This got me about an additional inch and a half which was enough. I could remove the shortners and change the seat post with the saddle already on it or go back the other way in 2 or 3 minutes.