Does my son need a return flight for visa-exempt entry to Thailand?

No he doesn't need a return flight, but will probably need an onward ticket out of the country (bus ticket to Laos will suffice)

100%, airline probably won't let him board without it.

Yep

Stock answer as asked many times here.

If you are entering on a long term visa, like a Non O or Non OA or Thailand Elite you do not need an onward flight out from Thailand.

If on a visa exempt entry or tourist visa you may well need to have a flight ticket out of Thailand within the amount of days that your initial entry allows, regardless that you can get an extension in country. It doesn’t need to be a return ticket, any ticket out of Thailand will suffice.

In theory immigration can deny you entry if you don’t but they rarely ever would, because they know you can get an extension.

Almost always the hang up will be when you try to board your plane. Many airlines will not allow you to board without an onward ticket within the number of days.

Some will let you. Many won’t. Short haul carriers probably won’t care, but for long haul carriers they probably will. If you don’t have an onward ticket it’s the carriers responsibility (at their expense) to return you to your point of departure should immigration deny you entry for some reason.

You probably don’t want to be the one scrambling to buy an onward ticket at the check in desk. Best option is a changeable ticket, and just change the date when you’re here.

Alternatively you can Google “onward flight ticket” and you’ll find a plethora of companies willing to sell you one for around $10. A real ticket, Phuket to Singapore, would be approx £25/$35 depending on the time of year.

You’ll probably get a number of replies saying “I managed to enter with no problem, therefore you won’t have one either.” Well you won’t find out which way it’ll go until you get to the check in desk.