Unfortunately, the only way to play your digital games on another Switch is to deactivate one console and activate the other. Your son would not need to be signes in under your account, you account just has to be active on the one he is playing on. Then when you want them back, you deactivate his from being your primary and change back to yours.
It sucks, I know. But it is what it is. Personally, I think the family plan for Switch online should bypass this and allow him to play your games without your account being active. If you are tied to the same plan, it is associated with your account already. Hopefully they announce something like this soon.