What should my son do with his Non ED visa if he is leaving Thailand for another country?

If he entered with a 90 day non-ED visa or converted to a non-ED visa while in Thailand and is still on the initial 90 days of the visa, then he he doesn't need to do anything.

If he was on an extension of stay (the stamp in his passport says "extension of stay") then he signed a document that said he would notify immigration of any change in his status. That means he is required to cancel his extension if he stops studying. This can ONLY be done at the immigration office he got the extension from. If he does not do this then in the future he will not be able to get another long-term extension in Thailand until he cancels his old ED extension, and he'll also be fined 500 baht per day that he didn't notify immigration, up to a maximum of 20,000 baht.