My first advice is not to worry about
this too much. Remember that the vast majority of people with psoriasis never get arthritis. Those of us with PsA often had the skin diseases for a long time -- 20 years in my case, before getting joint disease. Next, remember that the treatments for skin and joint disease have improved greatly in the past few years, so during his lifetime it may not be a problem if this does get worse. So message one, don't worry.
Message two, a little more specific, is that he should know that the ointment he uses for his knees should never be used on the face or genitals. If he ever gets psoriasis there, he needs a different, and much milder medicine. Kids are shy, and might be tempted to self-treat sensitive skin with the medication he has, or even try out his knee medicine on a facial pimple.
My third suggestion is, to the extent you're both worried about this progressing, keep a watch on the finger and toe nails. Joint pain can be from many things, and it doesn't make sense to run to rheumatologist because his knees are sore one day. Most people I know with PsA had changes in the nails first.
Good luck, and don't worry. It sounds like his case is very mild.
Healing well with psoriasis, PsA, and hypogonadism, and happy to share my experiences.