Mini cannot chew/swallow his hay - again!

Exactly one year ago, just after having his teeth floated, my 18 y/o mini suddenly began slobbering and was unable to adequately chew and swallow his local grass hay. He was also unable to chew/swallow finer grass but could eat some coarser grasses and herbs. Of course, I blamed the floating. Four months followed of vet visits, xrays, tubing w/camera, various pharmaceuticals. His soft palate was swollen but no reason could be found. My vet consulted with other vets and with professors at Oregon State University and Washington State University and no solutions were forthcoming nor had anyone experienced a similar situation. I offered and soaked a variety of hays; alfalfa was easiest for him but he still left most of it. He could eat pelleted feeds, though the bottom of the feed pan looked like soup when he finished. In February, the condition began to resolve on its own and since then he has been just fine, aside from a bout with laminitis unfortunately brought on by all the unusually rich feed we gave him in order to keep him eating.

To my dismay, last week I noticed quidded hay in his stall. This time there was no dental work involved or anything else out of the ordinary. From then on, every day he has gotten worse until once again he is unable to chew and swallow his hay, even though he tries very hard. He is still able to graze, though grazing appears to be becoming more difficult as well. It has been very dry here. My pasture is very small and not very good; my horses are primarily hay-fed (the other two are having no problems nor did they last year). Since this occurred almost to the day from the problem last year, I now am thinking time-of-year-related toxins/allergies but I can find no information online that fits his symptoms.

Does anyone have any experience with something similar?

THE ABOVE WAS POSTED ON THE HORSE NUTRITION FORUM - I DID NOT REALIZE THERE WAS A SPECIAL FORUM FOR MINIS.

UPDATE: My vet thinks that yes, it is probably an allergic reaction to something and yesterday we started him on hydroxyzine. This morning his grazing was much better than it was yesterday evening, and he was able to swallow mouthfuls that yesterday he would have spit out. Still cannot eat hay and is very sloppy eating his pellets. But at least he is able to get some food intake.

Did you know he can live completely on Senior's pellets? It provides everything an older horse needs. Is the vet certain the mini isn't having tooth abscesses? A friend's horse went thro like your mini and it was an abscessed tooth. It was extracted. Once his gums had toughed up he began to gain weight and stopped quidding. He does need his teeth floated more often now as they will slowly fall toward the hole. He's was about your mini's age. Sometimes the infection under the tooth will blow out the gums and all seems fine but in time it will build up again.

Thank you for responding. I got a bag of senior pellets yesterday. He isn't crazy about them - would much prefer to munch on grass or hay - but yes, I know he can survive on them if need be. Thanks also for the info about abscesses. We suspected an abscess last year but it turned out not to be one. What is so strange is how the same symptoms arose two years in a row on virtually the same day! That's why we are thinking an allergy.