Unfortunately there's conflicting information. I'm posting the link to my claims source of information below.
Different sources have different information though and some sources support your claim... Including Wikipedia. I'm going to give on this one as it seems most likely that you are correct.
I don't have time to check and update my comment now. I'll return in the near future with citations to either confirm what you are saying or continue the discussion. 🙂
Multi-use allows unlimited entry for 6 months with each entry being a maximum of 60 days by default.
Considering you can file a 30-day extension to any non-expired entry stamp at immigration, the actual correct answer is if optimally used they can be in Thailand for up to 9 months on this visa.
respect about bringing citations. I have had it clearly explained to me a couple times that The number is 3, however, once you gave citations I double checked and your info is correct. And about to go and edit any comments I made with incorrect information.
Considering that he was smart enough to ask a good question and learn that the correct answer is actually 9 months if optimal utilized... He very much should be allowed to be out without his parents.