I'm going to pick up my son from out of state and I'm going to buy my round trip ticket. How would I buy the ticket for him when he's only 3? I know you need to be a certain age to fly unaccompanied, which is how I need to buy his ticket, but how do I make the purchase if it thinks he's traveling alone?
Since you say "out of state", I'm going to guess that this is domestic travel within the United States.
At the major US airlines, a domestic round-trip ticket is generally the same total price as the two corresponding one-way tickets. (Years ago you could expect a round-trip ticket to be cheaper, but that's generally not true anymore.) So you can book a one-way ticket for yourself from A to B, and a separate one-way ticket with two passengers (you and your son) from B to A. The total fare will likely be the same as if you had bought a round-trip for yourself and a one-way for your son.
This has the added advantage that you can use different airlines for A->B and B->A, in case one airline has a lower fare on A->B and a different airline is lower on B->A.