But that doesn't really address your problem. The landlord is responsible to the IRS to report income. He/she has no obligation to provide any of that information to you.
When you say that the company you work for "pays most of my rent," that sounds as if the company reimburses you. If that's the case, I fully understand why the landlord won't fill out a W-9 or provide a social security number. I've had plenty of tenants who've had home offices. But in all cases, the tenants paid me (they are the tenants, after all). I report the income on my taxes. I have had no idea whether they're reimbursed, or in what amount, by anyone else. That's not my business or my concern. And if some company who employed or contracted with one of my tenants asked me for that information, I'd refuse, too. It's none of their business.
As a practical matter, too, it could get your landlord into all sorts of trouble that isn't his fault. Example: Let's say you pay $2,000 in rent. Let's say you receive $500 a month for your home office. Right now, the landlord is reporting $24,000 in income from the rental. If your employer also reports paying the landlord $500 a month ($6,000 a year), now the IRS will have documentation showing that your landlord received $30,000 in income, but only reported $24,000. In fact, he didn't receive $30,000 in income.
It might be different if the company you work for makes payments directly to the landlord.
But if the company is reimbursing you, then they have all the information they need to show the IRS their payments. They may want a copy of your lease, which you can supply them. Or they may want other evidence that you actually have a home office, in order to justify the expense if the IRS asks.
Sorry, but your landlord's position is justified. You need to work this out with your company.
This is a good question for your accountant or tax professional. you should also check with your company to see if they will intervene since they are the ones who need the information. They may have a representative or attorney who handles these situations for the company. All the best.
Your Castle Realestate