3yr old eats junk food & won’t eat real food

My stepson never really ate what everyone else ate he would only eat hotdogs, mac & cheese, pizza, french fries, chicken nuggets. Now all of a sudden he refuses to eat those things. It’s bad enough that he doesn’t eat any fruits or vegetables occasionally a half of a banana. Today My bf made him pizza & French fries and he didn’t want it. I watch him during the p.m. hours while my boyfriend goes to work overnight and I offered it at dinner & he still didn’t want it. Lately he hasn’t been wanting to eat any of the food he likes, Just junk food. I go to look in his overnight bag & there is a bag of Cheetos, Pringles, pop tarts, fruit snacks, oatmeal cream pie‘s, chocolate pudding,with a Tupperware of chicken nuggets and a kraft Mac n cheese that you fill with water to put in the microwave. That is the food he eats literally eats daily. I told my boyfriend that his mom is just feeding him junk food that’s why he will not eat wat he usually likes and my boyfriend said that maybe she’s going through something where she can’t feed him a proper meal🤷‍♀️. This is coming from the same mom that said if he’s whining all the time just give him some candy. And I’m pretty sure they don’t brush his teeth but I do when he’s over my house. Like what am I to do about this? Should I just leave it alone & maybe this is a phase he’s going through ? Or just leave it alone for the simple fact that his dad doesn’t care and his mom is “ maybe” going through stuff