if you have any type of insurance, homeowners or auto, I would talk to them because if there is coverage they have duty to hire attorney for you (your son).
If you have no insurance, you will want to find an attorney with experience handling injury defense claims. Be forewarned, they are not cheap!!
One issue is whether you are personally liable for your son's accident, which I am assume you did not personally cause, but they are asserting you negligently entrusted your son with the ATV. This can be a very complicated legal claim so you absolutely need to have an attorney representing you.