If they or your son can do it from home before departure, it's easy enough to do on the website using a laptop or desktop and then printing out the form. I had no trouble* doing it that way, but I was able to do it in my office before departure because I was going to be away less than 7 days. I'm dreading the day I have to attempt it while OS on my ancient ipad2 or on my iphone with no printer attached.
*No trouble apart from extreme frustration with the govt IT idiots who think it is a good idea that you have to upload vax certificate, but then you have to separately and painstakingly type in all the details from said vax certificate.