In my experience OTTB lose condition fast and they lose condition and weight when they aren’t being worked.
When I’ve had to feed up an OTTB I’ve done free choice very good Timothy plus at least 5 lbs of alfalfa a day. Then the same small beet pulp mash to carry a vms and flax as I use on the easy keepers.
If they are still underweight there is a cool calories product here that’s basically alfalfa meal and vegetable oil pellets. Very palatable and high quality.
The key is very good hay. If your hay has the baseline protein level of about 10% then some alfalfa will give you the boost of protein you need.
I agree he should be eating upwards of 20 to 25 lbs of quality hay a day. 30 lbs is not excessive for a TB. Get a fish scale and weigh it. You can stuff a large hole haybag so he has hay in front of him 24/7.
I don’t know TCS. We don’t have it here. I think from what I’ve read on COTH that it’s a quality feed that is forage based, but probably lower protein? I don’t myself think any horse I’ve come across needs 12 lbs of any bagged feed unless he has no teeth and it’s a forage replacement. I would cut the bagged feed and give him free choice very good grass hay with about 30% of total hay as good alfalfa.
I switched over to commercial 2nd cut Timothy from random cool climate grass hays several years back, and it has been the best thing ever for both hard and easy keepers.
I also really like the results of a comprehensive vms with plenty of copper and zinc.