However, it's entirely possible outside of the rules.
Absolutely nothing stopping you and your son creating your own units and playing the game however you want to play it.
I know there used to be unit point calcuator systems for older editions of the game, not sure if there's a similar thing going around for this edition.
It might take a bit of playing around, using current codexes and points costs as a basis. Use an app like battlescribe for those current unit points costs and costs of upgrades, weapons, etc, if you don't want to buy a codex.
It'll take a little bit of work and experimentation but you'll be able to work out roughly appropriate points costs and you'll be able to build two fairly balanced opposing forces.
If it's any consolation or encouragement, your home made, amatuer attempts at appropriate points costs and balance likely wont be any less balanced than the current official product
Automatically Appended Next Post: Any thought on the types of custom units that you might be intereted in building?