My son has overstayed his visa in Cambodia. He has no money. What can I do?

First, requests like this are very often scams by people impersonating relatives, so make absolutely sure it's actually your son contacting you. If he has access to email, that means he also has access to a mobile/computer that can do a quick Skype call so you can verify it's him. (Yes, Cambodia has tons of Internet cafes and good mobile coverage.)

If it is him, find out how much money he needs. The cost of overstaying your visa in Cambodia is $10/day: https://www.gov.uk/foreign-travel-advice/cambodia/entry-requirements (thanks @pnuts!)

He may also need a flight back home. You can buy this on his behalf, or get him to buy one locally. Beware that one-way international tickets can be very expensive, it pays to shop around and look at return flights (and throw away the return).

Then, if there isn't an easy obvious way to send him the money like a transfer into his regular UK bank account which he then accesses from an ATM, find the nearest Western Union outlet and wire him the money. They'll give you a code that you can pass to him, which will let him withdraw the money at the other end. Note that WU fees are kinda high and there are many other ways to do this, but for a one-off transfer this is likely the easiest option.

Don't bother contacting the local UK embassy unless he's been arrested and thrown in jail, in which case things get more complicated. (And be extremely leery if "he" wants money to bribe his way out: odds are it's a scam, or that he's being scammed.)