When I say "tandem" most of you will think of something like this-
but there are other ways to have two on bike.
The top tube is always available should your son feel like he needs a nap.
Speakers are always a good spot to perch yourself on, especially if they are on the back of Deep's Trikeasaurus (2.0).
There is always the possibility of grabbing on from the side. No one said you have to always be behind.
Sometimes tandem is just a suggestion.
Do you ride with a "plus one"?