If only one of the couple has lost their job and the other is still working full-time (30 hrs pw or over), then they won't qualify for Rent Supplement. Even if the other person doesn't work full-time, they may not qualify based on the household income (including the son) and the rent paid compared to the maximum rent limit for the county & family size.
I would demand that they continue to pay you the full rent monthly, and not let it slide while waiting on a decision - there can be a backlog on decisions so you could find that several months go by before they're advised that they're not eligible, and you're out of pocket.
You will have to sign the application forms (or if you have appointed an agent, he/she can sign) as part of the tenant's application, and, you must supply your PPSN, which is recorded and sent to Revenue each year along with the amount of Rent Supplement paid.
As you're receiving Rent at the moment, have you not sorted out any tax issues already?