Aleksei is a male Russian name. I don't get this fear of "too feminine", it is an old Russian name. Besides, he could always use Alek or
Alex as nn.
But
Sasha and Mischa are nn, I would name a boy
Alexander and
Michael or Michail and use
Sasha or Mischa as nn.
"You may give them your love but not your thoughts. For they have their own thoughts.
You may house their bodies but not their souls,
For their souls dwell in the house of tomorrow, which you cannot visit, not even in your dreams."
(On Children by Kahlil Gibran)