There are advantages and disadvantages to doing this. You have a vested interest and will bring more experience than a freshly minted CFI. An area that seems to be overlooked in primary flight instruction is a good briefing prior to flight. You could really improve the learning curve giving a good briefing. You have to ask yourself if your expectations will be higher because your teaching someone you know. I think if you cover the ACS thoroughly you will be fine. It could be a great experience for both depending on how well your son responds to your advice. I have two teenagers and would not even think about giving flight instruction to my oldest one because we are way too much alike and personalities would not mesh. Good Luck and have fun if you decide to do it.