Absolutely he knew, but depending on the relationship dynamic - he could be like any other gaslighted abuse victim and she could have wholly convinced him that it was his fault. Battered men are just as vulnerable to the 'you MADE me hit you, YOU were being the abusive one, your WORDS did this!' tactic. I know one man who was beaten with a baseball bat, burned, had fingers broken, etc - and he oughtweighed his partner by almost 150 pounds. Every time, she convinced him he caused it, he started it, they had an argument and it was just her 'reaction' to him. My own husband was once in a physically abusive relationship and still has scars and he was made to feel the same way. It happened because he 'made' her do it.
So keeping that in mind, if he believes she became physically violent with him because of something HE did, and he may believe she wouldn't become violent with a child because it's his fault and he started it. If she has also gaslit him to the point that he thinks his concerns are crazy then she probably would be able to do the same re: Lucas, or also frighten Lucas into not wanting to tell Daddy (but she didn't talk about not telling other relatives, so he tried to get help elsewhere).
It sounds like I'm making excuses for him looking the other way, and I assure everyone I'm not - but I've seen so many times just what long term psychological manipulation, emotional abuse, and physical abuse can do to victims. I also would not be shocked if there was this level of abuse going on in that relationship - if you can direct it at a child, you can certainly direct it at an adult.
edit: In the case of the man I mentioned, it took 3 years and multiple attempts for him to finally leave his abuser. He'd be almost out and she'd figure out a way to convince him he was the problem again.
