Is there any good way of preventing my son from looking at porn sites?

Centralizes station: I am a firm proponent of the family room computer rather than a personal computer for kids.A variety of software programs allow parents to block sites with a variety of content. This plus a 2 hr limit on non-school computer time should provide you with a modest amount of control. Be aware that all parents may not have the same concerns, and kids may seek out access at another site.

Internet filter too: I agree with my colleague. Also, there internet filters that cannot be removed with a password. One i know is called, "clean internet" which the filter is through the company's filter rather than through the personal computer. If the filter blocks a good site, then you can ask to have that site released pretty easily. This is a christian company but there are others that are not religious. Be well.