How Can Blisters In The Tongue Be Treated?

My husband has blisters on his tongue and has had them for over a year. We thought it was his dentures and kept replacing them. We were told that it was not his dentures. He has seen about 3 doctors. The last one told him to take vitamin B folic. He has been taking that for 2 weeks now. No improvement yet. He does not wear his dentures except to eat soft food and is getting very depressed about all of it. He was checked for cancer but no cancer. What can we do about it? Is there anyone else we can see about? Very frusterated.