Few Days Overstay

I have a job here in Thailand and get my yearly visa extension without any problem. I have a daughter 20 yo and son almost 22 now. They both have also been geting extensions through my job in the past couple of years. But just yesterday when I appplied for extension, they did not approve for my son. The reason they said is that he is 21 years of age.

Ok, well my son has been offered a job recently which he is now inclined to accept.

The problem is his visa is up this May 1st. We figure he has 2 options, one is to go for a quick border run and get a one month visa on arrival. Later when he gets the request letter and papers from the school, he can apply for non-B.

The second option and my question is if he instead overstay like 5 days, besides paying the fine will he lose the non-O visa that he has or had? Can he apply for 1 year extension with the same non-O visa even if he had overstayed? The company owners willing to hire my son said they can provide the papers necessary within a few days as they are accustomed to the process.

We will of course prefer the second option if it is indeed possible. That is my question, is it? Thanks!