The problem is Incredimail - a horrible piece of software that is totally pointless unless the recipient is opening the mail in Incredimail.
Incredimail is difficult to uninstall from a pc, but I strongly recommend you persuade your son to get rid. A lot of email clients identify it as spam which will make his life difficult if he is emailing businesses and companies.
Incredimail belongs to the same family as coolwebsearch, another malware nightmare.
Another problem with it is if the recipient is on a capped download tariff, the email is using a disproportionate amount of bandwith so could prove expensive.
Google 'incredimail' and see for yourself the amount of hatred aimed at this program.