Okayy. He is my first horse. I've been riding for 4 years (lessons) and have leased before...We think the woman who sold him to us possibly might have had him drugged (or it was because he was undernourished) because he was nothing like he is when I first bought him...
If he was undernourished and is now well fed and underworked that could certainly be adding to his behavioural issues. Can you tell me what he is being fed and how much work you are currently doing with him?
As for being drugged, before you jump to that conclusion, let me share a story with you:
I recently moved my own TB mare to a new barn. For the first week she was an absolute angel. Then she decided one day that it was just all very scary and began to act out in various ways. It is now almost five weeks since the move and in those following weeks I have worked with her consistently to re-focus her attention to me first, and environment second. Slowly and surely the results are becoming apparent, but it takes work!
I am sharing this with you because if I didn't know this horse, and know for a fact that I didn't drug her, I would have sworn that she was drugged given the initial calm attitude followed by a complete behavioural change. It can, and does, happen.
It is going to take time and patience to work with your boy, if you do not have a 'bag of tricks' so to speak in your training repetoire you will certainly need to begin a quest to find such techniques. Parelli is one place you can learn from, there are many others also. I would suggest that having an experienced person, preferably one that has dealt with OTTB's before, is going to be the best way forward.
All the very best with the training and the thrush recovery and keep posting on here for advice and ideas, there are plenty of people that will be happy to help!