Stuck between a hard place and the biggest rock.

[MrBond]

"As for the custody of my son, that would be unfair to grant her more time than to me. It's not because she withheld him for so long that it means she cares more. For what reason should she have more rights than me?"

Because she has shown to be the more responsible parent since he was born.

"For the moment, she's at her parents, and therefore her mum takes care of our son 24/7."

You have no proof of that.

"But I won't give up on him, and we'll keep asking the court for 50% custody immediately."

Which I know you're not ready for.

"If she really can't support the idea of being separated to our son, there's a very simple solution, and I'm open to it: Move back with me! Simple, really."

Or third option: she goes for FULL custody and allows the court to determine how often you can see your son because she has proof of you being an absentee father. This is probably the option she's going to take.

Lose the attitude.