Once a cheater always a cheater?

My boyfriend and his ex were together for 6 years. They have a kid. She cheated on him, they broke up he started hooking up with someone from his work. Him and his Ex got back together and he didn’t stop seeing the other lady. His ex found out and they broke up about a year ago.

He hasn’t spoken to the lady he cheated on his ex with for a year, she doesn’t work at his work anymore.

He has given me no reason not to trust him. I have no red flags. He’s not secretive at all.

My mom thinks I should leave him because “once a cheater always a cheater” but I don’t want his past to effect our relationship. He came clean and told me he cheated on her, he regrets is so much. I really don’t think he would cheat on me.

What are your thoughts?