First, connect with a group! Each group has its own standards and will have recommended models and vendors and sources of used gear. Many groups have loaner gear to help you get through your first year, so you don't have to buy everything at once. There's nothing worse than spending a lot of money only to find that what you bought doesn't meet the standards of the group you want to join! (Been there, done that; fortunately my unsuitable women's clothing can be recycled as linings and petticoats where it doesn't show).
Be sure to check the rules on age; your son will probably not be able to use a firearm or appear on the battlefield until he's older. Some places, that's 16. In others (such as Oregon, although not all groups) it's as young as 12, if you are with him.