Well, he would have been a Summers brother, through the fact that his mother was Katherine Summers - if what I speculated was true about Adam-X. (Naturally, I am not sure exactly what Fabian planned).
But I speculated that when Christopher (Corsair) Summers and Katherine were abducted - Corsair, as we saw, was thrown into the slave pits, while Katherine was kept alive. I speculated that either, tests were done on her for her genetics - or that D'Ken may have impregnated her - and took the embryo and grew it - detecting great genetic potential for mutation within her (which is why both Scott and Alex were born mutants). That infant/embryo was accelerated by Shi'ar technology, which would explain why Adam-X has no memories of his own youth.
He mentions that he did plenty of battle type arenas - this could have been to test his potential. Then perhaps, at some point, he had memories of Earth, from his mother's genetics (or just "sensed" that Earth was his home) and ventured to Earth, with little to no memories. Martin Strong, who finds him, calls him "Adam-X" and the rest they say is history.
I have an entire write up about him on my
site. I did the whole write up for his profile on the
Marvel site, as well as
Neurotap and
Martin Strong.