Hi PRK and Welcome to A1
From your post, it is not clear what he has held onto, but yes if they are assets of the company they are not his to keep and at worst could be pursued as theft.
Even assuming a court decided it was not theft, if they were paid for by the company, he would be liable for the tax of benefits received in kind.
Can't offer you too much advice without knowing more detail though, but if you can prove the assets belong to the company, you should have a reasonable case.