Oh look at how hard he tries! What a big heart that little guy has! He's a gorgeous little pony!
These ponies if worked with properly are generally very accepting of riders but only if handled well. Work with him lots, bend over his back and use your own upper body weight to start up the idea with him, put all sorts of things on him, especially if you are wanting to turn him into a therapy boy.
Once you he is accepting of pretty much anything you show him, and shown him to accept some weight(even just your own) adding a rider won't be an issue.
Good Luck with him, he looks like such a trier. Make sure to continue posting more pictures regularly. Excited to see how well he does.