Can I transfer games from my Microsoft account to my sons?

The Wavebreaker add-on is for in-game cosmetic items. It is linked to the Epic Games account it was purchased on, not necessarily your Microsoft account. Family sharing will not share the item with him.

If the item was not purchased on an Epic Games account linked to his Microsoft account, there is no way for him to use it on his Microsoft account.

Transfer a add-on is not possible, but you can add your account in his Xbox, so he can also use it.

Check this link for more info: My Home Xbox Setting on Xbox One | Share Your Xbox One

You should add your son in family account

Unfortunately you cannot transfer the game license.

if you go to setting > personalization >
my home Xbox > make this my home Xbox. you will be able to save you're a receipt of your Microsoft purchases on his console without purchasing all the games again.

Note: progress does not transfer through accounts.