If there was no mention of the utility payments at the outset, he can not charge you for them. Without a lease, he can change policy and bind you from that point forward, but you should not owe for the past utility bills.
Every legal matter is fact specific, and there are often nuances in every case. This is intended for comment only, and does not create an attorney client relationship.
He was not supposed to take a deposit without a written agreement. He absolutely cannot charge you for the utilities already used without that WRITING. He can however, charge you from this point forward. You should have contacted an attorney before pledging your $300.00 to a debt your landlord couldn't collect.