You can't really block all spam effectively, there is no feasible way to do this. Forbidding your son to send and receive e-mails is myopic, it's something he has to know these days, it's as ubiquitous as toasters and refrigerators these days.
A few years ago, people would still be ashamed of getting penis enlargement mails, thinking it had something to do with the websites they've visited. Nowadays, it's just that common that nobody takes offense anymore.
Teach him how to deal with these things responsibly instead of worrying.