My Horse Would Rather Eat Alfalfa Than Fresh Green Pasture

My OTTB hard keeper will stand by his stall all day waiting for alfalfa hay instead of eating off his 5 acres of lush green grass. Not to mention the fact that he won't even touch grass HAY. Typically I would stop feeding a horse alfalfa and just force them to eat the grass out of hunger but this horse will just lose weight and he is already a little underweight. Other horses have thrived on our pasture so I don't believe there is anything wrong with the taste of the grass. Anyone have any suggestions on how I can encourage him to forage off his pasture? Currently, I am feeding him 10 lbs of soaked alfalfa pellets a day to supplement the pasture, but he acts like he is starving when I come to feed him the pellets. And I would like to see him put on a little more weight.

You could have a multiplicity of issues going on. A pic of the pasture would be helpful. Are other animals using the pasture? Do you know the quality of the grass hay? If you have a #5 grass hay and a pasture full of weeds and noxious plants, the animal will probably have to wait on the alfalfa pellets. If all other things are good, feed the dry pellets by sprinkling them across the pasture where he has to go find them.

The pasture has been used for years to produce grass hay that is cut and baled. Not certain of the quality but it is definitely good looking grass with lots of clovers and thick and thin blade grasses mixed in. I used to have two horses that would graze almost constantly day and night on the pasture and they didn't require any additional supplementation to keep the weight on. In fact, I let him into our yard area where there is dandelion weeds and he picks those out to eat first!
I scattered some pellets out in the field to encourage him to spend more time out there eating grass and it seemed to help but he still spends probably about 70% of his day standing next to the barn waiting for food.
I am considering boarding a horse here that may show him how to graze more often for his food. Do you think that would be helpful?

So you have one animal? You have a lonely animal and its companionship comes when you feed it at the barn. It is waiting on excitement at the barn. He doesn't need "shown" how to graze, he would just rather wait at the barn than go graze.

I agree, he does get lonely which is the main reason I was thinking of having another horse here. Thanks for your input!!