@Niki85 two choices:
ask the guest to cancel and refund the remainder of unused nights
or
have a professional come in and treat. If the guest still complains, see the first option.
Fleas can rarely be treated effectively by over the counter sprays or chemicals. In my experience, if they have infested a home you need a pro to come out and kill them. You don't say if any of your other guests have pets or have had friends over with pets. If fleas got into the house they may prey on one person more than others, or the others may be desensitized to it as they are bitten often. Or your guest may be delusional. But he has shown you the bites so I doubt that.
Long term solution? Get rid of carpets. They are not optimal when hosting strangers as they trap allergens and critters. Some people see houses with carpeting as unclean no matter how often you shampoo and deep clean. I would strongly consider pulling it out in your guest areas and replacing with something that has a hard surface. You can use area rugs for decor or warmth. Good luck.
ETA-- you show cats in your listing. Do they go outside? Are they treated for fleas? I think your mystery has been solved....