you can wait until the last day, but if you are missing some document or there is some issue, you're going to go on overstay. Better to do it ahead of time to account for any potential issue
I'm not judging. I'm being blunt and telling it like immigration sees it. Coddling is not going to help this person get into Thailand and will only result in them getting denied entry and possibly losing thousands of dollars.
180 days in 2024 is not a holiday. That's living in Thailand. The fact that you have a son here is not relevant. You are not entering on that basis, you are entering as a tourist and trying to live here as a tourist, which is abusing the system. They've told you now that it's unacceptable. So you either need to find another option, or you need to act like an actual tourist if you want to enter as one.
Dozens of people are being denied entry daily for exactly that reason. After they switch visa exempt to 60 days, they no longer like people using it often. And they are denying a LOT of people because of it.