What Causes Blister Sin Mouth After Tonsil Removal Surgery?

My son had his tonsils removed 2 months ago. Now he has two small clear blisters on the back of his mouth, not in his throat. He has never had these before. I don t know if it relates to his tonsils being removed or not. He says they re not painful just bothersome. Any idea what this is or what has caused them ?