A work Colleague has been offered a position in China which he has agreed to, he will be leaving the company here and as such his work permit and extension will soon be ended.

He has a Wife and Son here who will stay until the end of the next school term, they are both on extensions of his stay but as he is leaving will theirs be cancelled as well ?

The extensions are due for renewal at the end of March and as he is not here working I assume they cannot renew their extension and must revert to a visa of some kind ..........

What do they need to do to stay legal for another 4 months ?

If they are staying as dependents of him, their permission to stay ends on the day that he quits his job as well.

Since the child is going to school, he can get an extension of stay based on his education. The mother can than stay as a dependent of the child, but must be able to show 500,000 in a bank account in her name in Thailand.

Alternatively they can get a double entry tourist visa from a neighboring country.

The extensions of stay end the day his employment ends as they are currently based on that - a conversion to student for student and accompanying parent for wife would require 500k in bank account for 30 days.

Depending on nationality tourist visas might be an option.