You have stated 'I'm fairly sure he stole some towels'. You need to be more than fairly sure, you need to be certain he stole those towels before you accuse him as he could turn around deny it and put in a complaint against you.
Do you clean the property yourself before each guest? Were the towels there when you checked him in? Did you check him out? Has anyone else been in the property between checking him out and you noticing the towels were missing? If you can answer yes to all those question the go ahead and e-mail him via the Airbnb platform. Ask him where the towels are first of all. Inform him you have searched for them as soon as he checked out to clean them but have not been able to find them therefore you intend to open a claim against his deposit via the resolution centre.
Then get a couple of 0n-line quotes for similar or same towels, sent the claim along with the quotes via the resolution centre.
He more than likely took them but you need to be certain before you make allegations of theft against a guest.
BW, Ama