When he first behaved like this, did you bring him home? It sounds like he has learned that napping leads to him coming home. If you have had is back, teeth and tack checked then it really does sound like him misbehaving. Ex racers would be used to going out in groups, so going out alone is quite unnerving for them. Maybe something did frighten him initially for him to go from being good to behaving like this but if he got away with coming home the first time, he has learnt what to do to get home quick. You will need to be very firm (though in my experience, using a whip usually doesnt work on TBs, they just go off the map!!) and ride him through it, DO NOT let him turn for home. If you feel you dont have the confidence to ride him through this, maybe you should seek help from a professional or if you have a confident friend who would help you out then maybe they could take him out a few times and get him through it. Good luck!

hi my mare used to do exactly the same. unless he is too dangerous then all i did was just not give in and just go short distances and however much she reared or resisted just kept trying until they give in and now she hacks alone perfect.