How about "Half second cousin", Veron! Anyone else agree with that?
Easily clarified by a simple calculation.
Genetically speaking, you're not related to the person who contacted you because your mutual connection, the son by a previous marriage, is not genetically related to you. However, by marriage, you are a half cousin (in law) of some degree, but it is not second.
It is first half cousin (in law) twice removed. Your grandmother and the gggranny's brother's grandson are first cousins. The person who contacted you is two generations descended from his grandfather and his half sister, putting him in the same generation as you are.
Here is a diagram that my lack of a social life gave me time to construct:
4xgmother
3xgmother her brother
ggmother ----
gmother (cousins) grandson == half sister grandfather
mother ---
Headbanger Veron Contact person
There will be an examination at the end of the school term!
I was able to do this because I once had to figure out my relationship to someone who turned out to be a half fifth cousin.
Regards,
John