Yikes. Six pounds of sweet feed makes me shutter.
I think you'll find that most of us steer clear of sweet feed. Main reason being the sugar. I would suggest that you look into switching the horses over to a low starch/low sugar pelleted feed. Tribute, Buckeye, Triple Crown etc.. any of their lower sugar pelleted feeds. Some people like & use Purina Strategy or Neutrena Safechoice, which are easier to get and can be found at your local Tractor Supply Co others steer clear of those and other Purina feeds as well.
Are the three other horses receiving six pounds of grain a day under weight as well? If not, and depending on the size of your horses that is more than likely to much grain. You'll find we also repeat free choice good quality hay
, supplement with a feed. I personally have never fed as much feed as what the back of the bag tells you to for your average horse. Some horses here get as little as a couple of handfuls just to help them eat a powdered supplement or their flax seeds. One horse here always needs "more" so he eats about six pounds of feed a day, split into three meals. My mare nursing her week old foal has free choice access to two kinds of hay and receives 5 pounds of beet pulp and 6 pounds of a low starch feed split into three meals a day, among other things, if that gives you an idea that they are all situation dependent there is no one size fits all feed type and amount.
For getting weight on your older fella, free choice quality hay is the key and there is NO way around that. Beet pulp and a senior feed are also good things. Slowly work him up to what seems to be putting weight on him. If he needs more of a push alfalfa, corn oil and rice bran also help with weight. Do you have a veterinarian that can offer you advice?
And in regards to your hay situation, if the hay is so dried and brown that they wont eat it I suggest you buy new hay. It doesn't sound like they could possibly get what they need from that hay and they might "know better" than to eat it. If three horses wont touch it and you have to shake the dust out of it... buy new hay.
This is all JMO. Hopefully someone that's more of a feed expert than I can come along with some advice shortly