<<How do I go about buying three open jaw tickets and one return trip ticket (for son) on the same return flight?>>
Book the same flights where you want those the same, but as two separate transactions, you can't buy different flights on one ticket (strictly speaking, one transaction, it will still be different tickets even for those booked together). You can then do whatever is needed (phone airline, do online, to arrange seating near to each other. Sitting together is not connected to buying tickets together, after all if you were flying alone you'd still be sat next to someone and they didn't buy their tickets at the same time as you :-)
<< I'll probably use Kayak.>>
Do they sell flights now? I thought they mostly (or only?) pointed you at a variety of companies that do the booking, some of them , unfortunately, ones with less than stellar reputations with much wailing and gnashing of teeth about in here. Some of which will *say* that the price is X but actually after a few days of hassle it turns out to be Y, where Y is a price higher than you could have bought on the airline. But with added hassle.
Use kayak to find out which airlines suit your needs. Then book direct with the airline.
I'm going to disagree with LeicsLad and his cute beagle, dont phone the airline, too many possibilities for details to be taken down wrongly and little proof who got it wrong. Do it online, have someone check as you do it.