I'm thinking of telling my biological son that he is adopted
What do you hope to accomplish with that?
We can discuss that when it happens.
It is wrong to lie to kids about being adopted. You should be honest or they might feel they cannot trust you anymore.
Yes it's against the law to lie about things like that.
I don't know the legal specifics, but I assume that should be one more reason for you not to lie to your son. There is really no reason for you to betray your child's trust in this fashion, and I hope I have convinced you not to do it.