I would suggest that you use a 30 or stronger sunscreen every day, and see a dermatologist to see if a medicine such as 5-FU would be helpful. The moderate sunscreens are mainly PABA-free because of the potential of PABA allergy. PABA in sunscreen has not been shown to be a carcinogen to my knowledge. There are sunscreens now that are chemical-free and contain micronized titanium or zincoxide. I would suggest that you see your dermatologist to help you develop a program of sun safety.
Dermatologists recommend covering all exposed skin with sunblock during mid-day exposure.
My son in 24 and has had three basal cell places removed, two at the hairline and one by his ear. He now goes to a dermatologists every quarter. Your son probably has a somewhat increased risk of melanoma, especially if he is very fair-skinned, and has a history of many sunburns during childhood. Examining himself on a regular basis and seeing his dermatologist every quarter will help detect any early cancerous or pre-cancerous growth. Currently there is research on the development of a vaccine to treat and prevent melanoma.