I won't claim to know anything about riding off-track TBs but I thought I would throw my 2 cents in.
This horse sounds a lot like a horse that has had his "plug pulled" and if it were me I would treat as such. Boots and BlueSpark touched on what I would do with him which is what I found works on these types.
Get him outside, don't pick at him and find something that piques his interest. You probably don't have access to these types of activities but working cattle or running horses(wrangling the saddle horses or gathering the broodmares and babies) on them gets them interested in a job and puts some life in them.
I do have a friend that was a jockey and used to go back east to start race colts every year and now takes off track TBs and makes cowboy horses out of them. Like Boots does, he doesn't give them the let-down time, he just starts taking them to work doing easy stuff at first like riding and checking cows, making meadow moves. It doesn't take long before they start enjoying their job, getting more broke and pretty soon he's roping off of them. He treats them like a horse.
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.
PLAYBOYS OKIE CODY "HOOEY" 10/21/2019