All great advice; I will add one more thing:
PLEASE have blood drawn to check his insulin and ACTH levels.
The TWH in my avatar was 16 when we went on our last organized ride. He was always a huge-motored horse.
Would start out in the top ten of 250 - 300 horses and finish the day in the top ten. Not because I pushed him, that's where he wanted to be.
On our last ride he paced himself different and we ended the day in the bottom 50 of around 290 horses. I was heartbroke and also attributed it to him getting older.
We had done some pretty intent butt-sliding in our younger days, so I "retired" him from organized rides but still kept trail riding either alone or in tiny groups to where he could rest when he needed it.
THAT was the first warning sign of insulin issues but it wouldn't be until he was 19 that I really got slapped in the face with a dirty diaper because I had no idea what metabolic issues were all about.
He is now 24+ and I am managing him with a strict diet, herbs, and NSAIDS because the metabolic issues exploded his hock/ankle arthritis.
Tennessee Walkers have been added to the Predisposed List for metabolic issues, so please get your guy tested.
I have three TWH's and two of them have insulin problems. They are essentially Type II diabetics if they were human.
Duke is pretty much retired only because our road is nothing but massive hills, the only flat spot is where we live at the top, so it's no fun to take him out and watch him go flying out the drive (like old times) and the minute he has to climb the second hill, he's wearing out.
If I could load him and ride him on the flat somewhere, he could go for a couple hourse but now that his hock/ankle arthritis has gotten so bad, it's really tough for him to get in the trailer, then back out of his front spot and turn around to walk out.
If your guy is healthy he should go at least another 8 years at the same pace.
Years ago, my Arab/Saddlebred that I raised from birth went "Heck Bent For Election", did his last organized ride when he was 27 and never wore down. Folks couldn't believe he was 27 - he was tough as nails and looked like he was 12. Not long after he got sick and the cancer took him at 29.
For my third time, please have your horse's insulin and cortisol levels checked. Good luck getting this figured out