Passport advice

Hi, I want to get a passport for my sons birthday who is 20 and he has a kids passport I believe from when he was about 6 years old so now out of date.
I've got him to take a photo at the passport photo booth, now I want to apply online for him but I'm not sure what I do with the passport photos he has?
I can get them signed by somebody but what do they write on the back of them as I'm a little confused what I need to get done next?
How many do I get signed and do whoever sign it need to write anything on the photos and on how many of them?
I can't find anything online what to do unless I actually go ahead and pay before I get the photos signed. Cheers for your help.

You write
I certify this is a true likeness of *******
Signed by the person certifying

Just on one photo?

You can do this online. There is a download form for the witness to complete/sign...then you can upload.
I THINK they then send a confirmation email to that person.

BTW do you not think he is old enough to get his own passport???

https://www.gov.uk/apply-renew-passport

They make it easy to apply (usually), just follow the step by step instructions.

Cheers. All sorted.