Hi Greg
Glad your son has taken up the hobby. It's how my dad started me off with my first set in 1958.
 By "Controller" I assume you mean Transformer? For right now the cheapest and easiest way is, If he is going to stay with the hobby, then pick up another transformer that has  multiple output(s). Lionel makes many.
What usually is done is, as the layout becomes larger or permanent, you either upgrade to a multiple source transformer like the ZW (Which has 4 outputs) or one of the others that has multiple outputs.  Most folks will put their accessories on a separate source, Another output is used for constant voltage Power for  your switches and yet another 1 or 2 outputs for train control depending on how many isolated track systems you have.  I have more than one transformer running and I won't get into that right now. It takes a little bit to correctly set that up. Lionel has videos on all of this you can watch.
Claude