Welcome to the community!
Without knowing what state you are in, I can not say for certain what the procedures are for removing a child from school to homeschool him. It varies not only by state but even districts as well. Some are simple processes with little hassle and others experience challenges to get this done.
Since you already homeschool, you likely have the contacts to find this information. Call the local school board or the person you deal with in homeschooling your daugter and inquire about removing your son from public school. They should have the answers or be able to tell you who to contact.
Instructing him on areas that you are weak on is another matter. There are materials out there that are self explanatory and there are also videos with full instruction. Your budget and personal choices dictate what is best for you. Would you like me to find some of the options so you can see what is out there?