Wedding Inviations

Typically, the inner envelope is where you indicate who you are inviting -- whether it is one individual and guest, or the family.

Since they are already sealed -- you could simply put
The Andersons (thus indicating the entire family) or
Mr. & Mrs. James Anderson (thus indicating only the parents) or simply
James Anderson (singular). Hopefully he will be able to figure out that he can bring a guest!

Good luck