First of all, I doubt that soma has all of drac's memories.
1) in AoS, if soma had gained drac's memory, he would have started calling Arikado either Alucard or Adrian.
2) in AoShe only says that he knows Arikado is there
3) Demitri says something in DoS that makes you wonder:
"Have patience, Arikado. Or perhaps I should address you with
your real name? When the boy began his dark transformation, I
scanned memories of his past lives. Make one move, and the boy
will die. You don't wish that, do you?"
I wonder if this means that if Soma learns the identity of someone significant to dracula (for example: Alucard) it would cause him to revert back to Dracula, effectively killing Soma.
4) In AoS, he only says that he understands that he is dracula.