First thoughts are the horse may have renal or other organ failure, or possibly ulcers if he's lost his appetite? I wonder if whoever owns him could have a vet come out to do bloodwork on him if possible.
He may not be touching his alfalfa for that reason. If he still has an appetite and is eating some or all of his senior and beet pulp, I would suggest feeding well soaked grass pellets instead of alfalfa with less beet pulp. I'm presuming these are soaked into a mash for him. If he's, say, ideally a 800-900 lb. Horse, he would need at least 4 lbs. Of soaked grass pellets 2x a day and at most 1 or 2 lbs. Max of soaked beet pulp 2x a day (much less if his organs are failing him).
Mind you, I'm just talking off the top of my head here and offering ideas, but it does sound to me like the horse needs a farm call.
Here's a link to a nice article with some info on renal failure - and personally I don't think it is all too rare: Diet for a Horse With Kidney Failure | eHow.co.uk
If there's any possible way to buy or borrow a scale and for those who do the feeding weigh hay and grains initially, that's a big help, seeing's how a flake can mean 3 lbs. Or 8 lbs., and so on.
Beet pulp ferments easily so that's something else to watch for too.
A microbial supplement such as FastTrack or whatever's available in your area is great stuff. Rice bran, another one. And adding a small amount of corn oil to his feed 2x a day may help. I'm assuming he has a salt/mineral block and water 24/7. If it's renal failure, some bute and an appropriate diet will help keep him comfortable meanwhile. I hope any of these ideas, suggestions can be of help.