First thing you need to do is consult with a lender.
Even though his income is $20K/year, his employment history, debt ratio, FICO's, etc. may impact the "bottom line" net worth he actually makes.
Once you get discuss this with your lender, they will be able to tell you how much you actually qualify for.
It'll save you alot of time in researching a home plus when putting in offer's, you are required to submit a pre-approval letter showing how much you qualify for.
Best of luck!