A separate account for him definitely. Then use the keyword filter. There's not really any other way other than supervision.
1. Direct instructions to eliminate porn from results. Others to add ... xxx, adult, tape, cam, sex, nude
2. You could also add file extensions to the word filter. Such as .jpg, .gif, jpeg, .jpe, .mov, .wmv, mpg, mpeg, .avi, .div, .rp, .hd, dvd, vcd, swf, movie, video, film, etc.