Save whatever is left of the product, save what is left of the packaging and the receipt for the purchase. Clearly photograph all of the above and back up the photo files. Contact a personal injury attorney in your area ASAP.
I am sorry to hear that your son is sick. But, without more detail it is hard for me to give you additional help. Most personal injury attorneys, like myself, will offer you an initial consultation free. I encourage you to do this ASAP.
A roundup of the best tips and legal advice.