I don't think Shashen would be Bowki's son! He'd be at least a great-grandson... with the "other parent" in each generation all having been pureblooded elves.
Shashen would be the 'memory of Shushen' in the sense that I think Shushen would be remembered by all the Sunfolk (after all; they don't have a "Now of Wolfthought", and for them dying isn't really natural, especially not since Leetah was born...)
So throughout the years Shushen was always remembered, and eventually Bowki's grandchild recognized, and during the pregnancy the parents just knew that their son's name should be Shashen.
Also: Bowki as a child may not have learned, but Bowki as an adult?