Honestly, I'd go look him over, lead him around, ask him to trot if you can. You know how a foal should move - just watch him and see what you think. See if you can do a deal where if he doesn't show signs of improvement in 30 days, you get to bring him back.
Honestly I think he's precious and $50 is a great price for a colorful foal. Can he be registered? He looks to have a decent build, and if he can be registered, I'd do it.