Greetings and Salutations;
I am sure others may have said this, and I hate to be disappointing, but, the fact of the matter is that you cannot do it. There is no rating system that can be queried to see what the content of a site is, and, there is no way to tell from the domain name. Any wall that would catch porn sites will also block content for pretty much all sites dealing with health issues - which is a net loss.
Remember that the Net is an adult venue, not a children's playground, so, children using it should not be doing so unsupervised. I would suggest that a better course of action then trying to use a sieve to bail water would be to take the following actions:
0) Have a talk with him, and explain that he will likely run across sites on the Net that he should not be dabbling in at that age. Tell him that he just needs to browse away from them. Also, assure him that if he runs across something that he finds disturbing, or needs explanation, that you (or his parents) will be glad to talk to him about them, and he will not be in trouble for stumbling across them.
1) Put the computer he will use in a public place. In the living room, not his bedroom.
2) CHeck the browsing history on a regular basis. If you find too many inappropriate sites in the history log, then, have a talk with him.
3) If he is 13, puberty is starting to stir, so, frankly, he IS going to be interested. However, if he has a good upbringing, and has been taught to treat women as equals, not just objects for sexual gratification, he is more likely than not to be put off by the sites.
4) Unless something is really off, the fact of the matter is that it is unlikely to destroy him to see folks naked, so, again, it gets back to #3 - treating women as equals instead of objects. Frankly, I would rather that a kid see a few naked women on the net then spend too much time on the myriad other sites that spew hatred and bigotry, and, call for violence as the answer to all problems.
5) Remember that if he really wants to go there, he will find a way, and may be good enough that he can cover his tracks so you will never know. It is far better to do the tough thing - which is to talk with him when he has questions about that sort of thing, do it in a non-confrontational manner; and promote communication.
Regards
Dave Mundt