Sorry, a little late to the party but: Check your horse first for saddle fit, chiropractic issues, and ulcers. Have his teeth checked as well! Those are the most common issues and they are all fixable. If your vet confirms the arthritis issue you can help with medication and supplements. When all that is evaluated it is time to get him some training. It might be that he just doesn't see the need to go faster and has gotten away with refusing before (he was sold on Craigslist, a hint that he might have some issues). He sounds like a nice fellow and could still become your dream horse! But don't work on behavioral issues until you have evaluated all possible physical problems!