Until you know for sure what this is by having your vet diagnose it, I don’t think anyone will be comfortable giving you advice on treatment.
Ask your vet if they are seeing anything this spring like vesicular stomatitis—this is my 1st thought when you mention painful blisters in his mouth. I wouldn’t ride him i.e. put a bridle on him until you know. He sounds very ouchy—hopefully your vet can diagnose quickly & an easy treatment plan will be made available!