In Illinois if the landlord has more than 25-units in the same complex they must pay interest to the tenant equivalent to what would have been earned at a bank. The landlord would need a W9 in order to file a 1099 when the interest is paid to you, but even if the interest was not substantial enough to warrant a 1099, the landlord is likely utilizing a trust account and banks frequently require a W9 to open these accounts.
Is there risk involved in him opening an account under my name? For
instance he has some sort of access to it I assume, what if he misuses
it in some way and my name is on the account?
I thought the typical approach would just be keeping the money separate via bookkeeping, or maybe opening an account to keep the funds separate, but not actually opening an account in your name. However in some states it is required to be held in a separate account, and commonly that is achieved via trust accounts. Illinois does not seem to require separate accounts, but it seems an acceptable approach regardless of whether or not it is legally required. So, the W-9 seems relevant, and I initially thought most of the other info was on the W-9, but DOB/Phone Number/Occupation/Government ID don't seem relevant. Phone is obviously a practical measure to contact you, and ostensibly they would have already used DOB/Government ID to confirm your identity, but it is odd that they are requesting those at this point.
If you've otherwise validated that this person is the owner/agent for the unit you are renting (since you're living there already this seems given), then the requested information is not too suspicious but the timing is odd and I don't know why they would need anything other than what is on the W-9. There's always concern about how sensitive information is handled, there is definitely potential for abuse/misuse.
The W-9 component is only common practice in states that require damage deposits to accrue interest.
Email does not seem like the correct place to request all of this
I agree, I would expect to fill out the W-9 and sign it and deliver it in person, but people are definitely in the habit of emailing sensitive information back and forth.
Why would he be starting this process just now, 3 weeks later?
The W-9 maybe wasn't relevant until damage deposit was paid, and maybe isn't urgent to have prior to move in, or it may have been overlooked initially. The information not relevant to the W-9 is more curious.
Since you are already living there, furnishing the W-9 seems reasonable, and you should ask for reasoning for the other info you haven't already provided. I would deliver the information in person, not via email.
This is all just my take based on experience as a landlord, but I don't own property or practice law in Illinois.
(765 ILCS 715/) Security Deposit Interest Act.