I have only read to first page so have no idea what advice you've been given but....
WHY Are you riding him? He is thin! 7 days a week? I recently acquired a few off track boys myself but riding is OUT OF THE QUESTION until they are at the proper weight.
Two flakes of hay daily, I do not care what the type, Will feed his left front hoof on a good day and is unacceptable.
Read up on what to feed Off Track Thoroughbreds, Google it. You will find page after page of "...Free choice high quality hay" "...Expect to feed HEAPS of hay" If you aren't able to offer free choice GOOD LUCK putting weight on him, Find a way to feed him properly, two flakes a day is what you feed a Miniature Horse.
Along with free choice good quality hay, He should be worked up to easily three, 3 QT scoops of soaked beet pulp a day. Three, 3 QT scoops of a good, quality, "Complete Feed" Such as Purina Equine Senior, Triple Crown Complete. He should be on a weight building supplement to ad calories and worked up to 3/4 cup of corn oil. Rice bran works great. Alfalfa cubes work great. He should NOT BE RIDDEN if you want him to gain weight, certainly NO Thoroughbred should be put to work under saddle on two flakes of hay a day?
Also.... Be careful of boarding stables and the barn manager feeding what she wants to feed. Nearly every boarding stable I go to these days has under weight horses, If they can get by skimping on feed they will, its your horse - BUY EXTRA AND FEED HIM if you have to. Boarding stables WILL NOT PUT WEIGHT ON YOUR HORSE FOR YOU it costs triple to pack on pounds, they cannot make money doing this. He will continue to loose weight with you riding him in bad condition and him not being fed properly. I've pulled skinny Thoroughbreds from boarding stables who claim the horse has a problem but they do not, It just costs them more to feed a Thoroughbred than a short stocky QH would eat and they get paid the same per month to do so, In conclusion the horse looses weight.