Divorces can drag on, and an awful lot of people start dating before it's final. I'm assuming lover-boy has his own apartment? I have no problem with people dating during a divorce (except for the fact that they may not have examined what went wrong to avoid making the same mistake). So my gut feeling is that it's stupid, not necessarily immoral.
That being said, the two of you know what's bothering her, she's feeling dumped anyway and basically...well, the two of you are just rubbing her nose in it. AT least, that's what it feels like *to her.* If Lover only has his son half the time, you would be doing yourselves a big big favor to see each other when the kid's not around. In fact, to continue to date *in front of the child* is inviting drama. It just is. I don't care how platonic you keep it, and clearly, neither does soon-to-be ex.
If your attraction is that strong, and you're meant to be together, a few more months of being discreet will pay off . I have a feeling though, that you're going to comment that you've already moved in with him. I think that's unwise, for the reason stated in the first paragraph, but it's your life.
ETA: Having read your other question: He dumped his wife in 2018, and you've been living with him almost a year. Yeah, his wife isn't happy. And you and he jumped into life together way too early.
"We hide any personal information from his son ( his idea) as he doesnt want him going back to his mom and saying something. So i feel so awkward when his son is there..." Should read, "awkward when his son is HERE" because you live with him. You're not hiding the fact that BF and you sleep together. The kid's probably not stupid, Honey. Neither is his mom.