What is the procedure for applying for a Retirement Visa in Thailand after an expiring Guardian Visa?

You can switch the reason for an extension at Room 103 at the main Phuket Town office. You cant do this switch at Blue Tree nor Patong. Probably best to go there and ask them for the handout to switch or check piv-phuket.com for the online list.

When will your son go back to Europe? The day he leaves will be the day your husband's permission to stay will end (assuming it's in advance of his extension expiry) so he may need to switch to an extension for retirement purposes earlier than you plan. He shouldn't need to apply for a new visa just switch the reason for his extension.

He will need to meet the requirements for this new extension and will need to have his bank funds in place for 2 months before applying unless he can get an income affidavit from his embassy (you don't say what passport he uses)

I would bet money that your husband has NO visa at all right now and if he looked at his stamps he'd see that it says "extension of stay permitted until" and has a date.

Your husband would not be applying for ANY new visa at all, just changing the reason for his extension of stay to one based on supporting a foreign child here to one based on being over 50 (retirement).

You need to GO to the Phuket immigration office (before he wants to change the reason for the extension) and get the hand out listing the requirements for him to get a year extension based on retirement. He would have to meet the proof of funds requirements, etc but he should have no issues changing the reason for his extension to one based on retirement