My name is***** and I'm happy to help with your question today. I need to ask you some preliminary questions to be able to help you.
On what basis is he claiming that you owe him storage fees?
He has had my fossil for5 years
I think you need to take a firm stand. He appears to in breach of contract - he agreed to do your work in return for a fee. Once he had completed that task he should have returned your fossils but did not do so. His failure to do so is a breach for which you could sue. There is no legal basis for him to claim storage fees for his failure to return your fossils - he incurred the 'storage' charges - not you. I suggest you tell him that unless the fossils are returned immediately you will take action for breach of contract.
Thank you, ***** *****!
All the best, Joe
Good luck. Any problems come back to me and I will help you further, for example, with a letter of claim. Remember to keep an eye on his movements in case he disappears again!