@Brent ******
He’s not on any visa. That passed a long time ago. He’s on an extension of stay based on being dependent on your extension of stay - you’re not on any visa either.
All that’s happening is that he’d be changing the “reason” for his next extension to being a student. It doesn’t require any “change of visa” or having to leave the country.
Many people assume that they’re still on a “visa” even after being here for years on “extensions”. They’re not. The initial reason you got your visa is relevant in some cases (like an AO visa - because of insurance requirements) but for most there is no relevance for changing the reason for an extension if you meet the needs of that extension. Ie you can switch from a “marriage” extension” to a “retirement extension” or vice versa at will provided you meet the criteria.
Switching to a “education extension” merely requires documentation from the school. Im not aware of the exact documentation but it’s generally a letter from the schools governors that they are attending, a timetable for their studies and a previous school report if applicable.