My 21-year-old son is in the very first stages of an outpatient program, so your question is something I may have to wrestle with eventually (God willing). It's not at all unusual for someone to struggle, sometimes for an extended period, to find employment, especially if they're searching in only one specific area. The answer to your question about whether to pay the rent at the SLE (I'm assuming you've done so up to this point?) depends on what has happened before, whether he is asking for assistance, how old he is, etc.
I know many adults who have had to take whatever jobs they could find until something "in their field" became available. That's just life. Frustrating, but not unusual. And working to support yourself any way you can (I mean legally and ethically) can be an experience that strengthens the psychological muscles that are so often atrophied in addicts. All honest work is noble.
Is this his first attempt at recovery? His fifth, 10th? Are you in a financial position to provide such assistance? And, even if you are, is it repeating a pattern of enabling? There isn't enough information about your situation really, but others who have been right where you are will have more insight than I.
Blessings to you and to your son. Ninety days clean!
