i had a young snake that developed a systemic infection as a result of her skin tearing during a shed. she had issues prior to the skin tear (issues that caused her skin to become weak enough to tear) and likely some sort of immunodeficieny, but that's what set off the systemic infection. she developed several fluid filled blisters like you describe, and the tears in her skin leaked pink fluid (the blistered/puffy areas were filled with this same fluid).

because of the eating issues it sounds like something was going on with your snake prior to the infection--something that possibly weakened him, causing the injury and infection. systemic infections are serious so make sure to follow your vet's instructions closely. did he happen to give you silvadene cream for the wound on your snake? that is a very good salve for burns and other injuries.

also, have you had a fecal done on the snake to rule out internal parasites?