I thought of a few more things overnight.
If you have/are going to have access to the internet, have you looked into survey sites? No one seems to be interested in my opinion, so I haven't had much luck, but my demographics are different from yours, so you might have better luck.
I do several paid to click websites. I probably make $30-40 every 3 months. I'm not getting rich at it, nor can I support myself on it, but it is a little extra under the table. email for more info if you are interested. Again, you will need internet access to do these and most cannot be done on public internet (like the library) you will need your own isp address.
When my daughter was looking at places to live when she was attending college a ways away, we came accross one place that exchanged room and board for doing farm chores. She wasn't that interested and we didn't look into it, but I thought it was a unique idea.
Some CSA's offer farm shares for labor. If you are not able to grow enough food for yourself/son, perhaps this would be a way to earn some additional. I'm not sure about how each one handles the under the table thing, though.
If you end up near a walgreens and/or cvs, watch their sales, as you may be able to get things like shampoo, toothpaste, etc for free or next to nothing.
Lora