Thanks for writing in to Healthcare Magic!
One important thing is if it developed suddenly say overnight or after a strenuous activity it is unlikely to be cancer rather it is just a blood filled bruise
and will subside in a week.
If it has been there for quite some time say over a week's duration and if aroused from a mole or he has similar lesion else where then he may be having a melanoma
particularly in refence to strong family history and you must visit to a dermatologist
as soon as possible.
It would be helpful for me to provide a stronger opinion if you could load a photograph in a follow up query.
If you have any further query please let me know I would try to answer it for you.
Wish you a great health!