Bm not cooperating- advice?

My SS(4), has some problems with his speech (he started talking very late and still can’t understand most things he says) and some behavioral issues like very intense emotions so he lashes out at BS(3) and sometimes SD(5) and not being responsive to us talking to him or when we tell him not to do that. He seems to be behind on his development compared to BS(3) and SD(5). He doesn’t like being around other kids very much and he’s very withdrawn and tends to be very emotional and skittish of everything. Me and their father have tried talking to BM about getting him tested to see where he was at developmentally but she blows it off and asks like we are being mean for even bringing it up. She seems to be in denial about noticing anything going on. I’m concerned that when he starts school it may be overwhelming for him if he does need special attention. BM recently started dating someone and has been much less worried about the kids and what they have going on so she has been allowing us to get them more.