calm and condition is a good feed to help gain weight, also try putting a tablespoon of vegetable oil in his feed, this should help him keep the weight on.
generally thoroughbreds are very thin, just make sure he’s worked enough to stop muscle wastage, and make sure you rug him up well enough so he doesn’t get cold, remember lack of fat = lack of heat.
if he’s not too fiery, also try him on a half hay/half haylage mix as well as his feed.
good luck!

Did you see if he needed his Teeth floated? If not have the Vet check on that.
My Paint mare was very skinny when I got her, so we put her on a diet starting out slowly.
Hay Cubes 10 percent of her body weight- Soaked in Hot water and molasses.
Senior feed in the green bag, for older Horses, but it has everything she needs in her diet.
A long with Hay pellets for both my Horses. I always have Hay available at all times.
14 percent Grain.
And the best fattening source is Corn Oil one cup in your Thoroughbred’s feed.
Please look at theses Pictures of Angel my Paint. Before and after pictures!
I wish you the best of luck. But please see if he needs his teeth looked at!

I have 3 ex racehorses and the first thing I do is have their teeth done, fecal to check for parasites and make sure that the wormer I am using hits the mark and then have them evaluated for ulcers. Ulcers could be what is causing the problems you are having.
I feed all mine basically the same thing, just the volume differs. They get alfalfa/orchard grass – 2 sections 3 x per day. They are turned out too and then I feed them Equine Senior as it’s low sugar/starch which prevents them getting hyped up but it has 14% protein and is high fat. I then add soaked beet pulp and corn oil and mine all look really good and have great coats.