If they happen to be communicating through text (as is common), then make sure your son saves all of the text messages. 

Just joinedNew hereLooking to connectWhat do all these posts have in common?

In Florida texturing drywall is extremely common.

Now thats common sense to have heat on when its freezing outside.

Now you can have a non-confirming unit that becomes illegal because someone calls the city and you get violations, however, it is COMMON PRACTICE in Chicago to rent out or stay in non-confirming units.

Is it common to have a pet deposit and added monthly pet fee?

Software is a common question on BiggerPockets.

In the beginning, I didn't even know what common terms of REI were. 

If such is the case, the tenants in the lower flat will hear the tenants in the upper flat walking around, and kids running around upstairs can amp up that noise.If the flats have a common forced air furnace (or if there are two furnaces in a common basement or attic), muted conversations will make their way between the units.

To my knowledge there are no HUD memos addressing this currently.More than anything, it is not common to offer rent discounts for certain professions, because most landlords want to rent their property fast to good tenants.