Why didn't you mention this before to the guest ? @Thomas332
Have you been providing him with regular changes of linens and towels?
does he have an access to a washing machine?
as you know you're not allowed enter a guests room during their booking so don't mention this.
I never let a room straight away after long term bookings but leave at least a couple of days in between guests @Thomas332
Talk to the guest and say the room would benefit from airing as it's starting to smell and ask him to open the windows.
the bedding will probably be fine as will the mattress (presume you use a protector)
leave the windows open for a few days after the guest has gone and give it a Thorough clean . @Thomas332