Tricky matter, all too common I'm afraid.
The problem is that you originally agreed to store his items--in many states that creates a situation where you have a duty of care for his goods.
However, is not indefinite in duration and you appear to have made good faith efforts to contact your friend and request he remove the items. If I were you, I would keep copies of all the emails, texts, voicemails etc. to substantiate your claim that you repeatedly contacted him and he refused to take care of his items. If you decide to donate his items, document what you donate, keep the Goodwill receipt to add to your documents.
A local legal clinic (contact your county bar association) might have volunteer attorneys familiar with Kansas law who can advise you on exactly what steps to take--perhaps involving the courts, against your friend as a further measure in protecting yourself from a later claim that you converted his goods (sold them for your own profit).