Acral lentigious is most common type of melanoma in African Americans and is on the palms or soels. I guess the typical thinking of chest and back (sun exposed) is a little different in this type? Among all demographics; melanomas occur the majority of time on the limbs (~36% lower and ~19% upper); trunk is ~27%... So based on that and him being of African descent we can choose palms.

For more explanations and pictures: https://www.aimatmelanoma.org/melanoma-risk-factors/melanoma-in-people-of-color/acral-lentiginous-melanoma-alm/

Melanin in darkly pigmented skin can provide an SPF (sun protective factor) of 1.5 - 4. Amount of UV light getting through is 1/SPF, so a SPF of 4 reduces UV radiation by 75%. So the areas without this protective feature are most at risk (palms/soles).

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2671032/

Acral Lentigious is NOT associated with UV exposure. So there might be another reason

I just think plams are not protected by the melanin and got this right.