Kris,
My sister does this kind of work. She charges by the month depending on how much work is involved on average. Granted that sometimes a business will need her more than other times. However, she is pretty much on call to them if their is any problems and issues. I know she charges one small business $600.00 a month. I like to kid her by calling her "Computer Geek for Hire". Jarrod would be running his own small business.
For my sister, it's a side thing. She runs the office for a AC/Heating business that her SO partly ownes. So, her hours are flexible and if a client has an emergency computer issue, she's available. Though, your son may prefer to charge hourly.
Incorporating isn't necessary. Especially, to start. There are advantages and disadvanges to doing that.I'm not exactly clear on the tax aspect, but I think he has to fill out a sole proprietor ship tax form and include it with his personal tax return. I would definitely consult a tax person for answers to those sorts of questions. Though, I would do the wait and see. When incorporating, one is creating a whole new entity and there are certain regulations that have to be followed. Plus, your son would be working FOR the corporation.
Also, depending on where you live, make sure he gets all of the proper local business licences.
