Hi and welcome to JA.Ray here to help you .
1. Do I have to release his belongings?
I would turn over the pcs to the landlord if possible.The landlord really has to deal with abandoned property.You as the new tenant really aren't required to do anything but to turn the pcs to the landlord.
2. Do I have to let him in the office?
You don't have to allow him access here to the office.I would suggest you send the pcs to landlord and let landlord deal with it.Landlord may well have line on the personal property but again once you send them to landlord you are out of it.
3. I was not told to send him a '3 day notice', so this was not done. He has 3 days left on his lease and its now Friday.
Again thats landlord's problem not yours..
4. Can I require that he has his lawyer document in writing his demands. Just to delay the release of his belongings?
Again you really need to send the personal property to the landlord and be done with it.It is not your fight, the landlord decides what to do with them.I would try to get a receipt from the landlord that they received these if possible.That ends your obligations to deal with it.You have no further liability once you do that.This is likely abandoned personal property here.
I appreciate the chance to help you today.Thanks again.