Firstly keep doing what you've been doing. Keep talking to him. Reassure him but dont lie to him, he already feels betrayed right now, you dont want to add him discovering you have lied to him to the mix (not saying you have), tell him an age appropriate truth. Also dont put down his mum or her new partner - I know it sucks big time, but you kind just have to take one for the team. If he mentions them, let him talk (again it sucks but let him). Reassure him when he asks if mummy can come home, "Mummy doesnt live here anymore, but Mummy loves you very much and Daddy loves you very much" or something similar. Reassure him that he is still loved.

Two books that might really help him get the concepts and allow him to raise questions on his own accord in a safe way:
- Two Homes by Claire Masurel
- The Family Book by Todd Parr