Uh oh :( all his joints are open and growing. His hocks, his knees, everything. A horse is truly not fully formed until 4 to 6. Even stock breeds. Whether they are growing in height or just broadening out. Not to say you shouldnt start riding until they are 5 but 1? Especially since he looks like a young yearling.
Even though many people start to ride at 2 I have seen so many horses that were pushed too hard too fast ruined by 9 or 10 getting hocks and stifles injected just to stay sound.
If these people just intend to do what their doing now, leading around for pony rides and eventually having a kid plod around he will hopefully be ok but demanding work will definitely break him down. He def shouldn't have a full grown man riding him at that size and age. He is very clearly still growing out and up. He'll probably be a hand or maybe 3 taller by the age of 5.
I would tell them to consult a vet. I doubt a vet would advise them to begin riding or at least not any demanding work. Maybe next time your vet is out for shots or something. Maybe a farrier would give them good advice bc they def know the beating a horses feet and legs can take. That way they can hear it from a professional in person. It sounds like they need a step by step plan since they are beginners such as once he weighs this much you can do this or once he's this age you can start riding for up to 5 days a week for this long doing these gaits/exercises.
Best of luck!