Autism or slowly developing?

My sister and I aren’t super close but I do help with my nephew very often. She isn’t super on top of it when it comes to taking him to the doctor or meeting milestones and stuff. He has never met any milestones when he’s supposed to. Recently her mom told her she thinks he may be autistic. She didn’t want to hear about it and said boys just develop slower. He’s 18 months and doesn’t talk at all. He said “Bow” one time (he was trying to say ball) and has never said anything again. Not mama, baba, dada.. nothing. He’s also extremely mean to other kids. It’s to the point where he can’t be around other kids at all because he is doing everything in his power to get to them to bite, hit, pinch etc. he’s given his cousin two black eyes so far. I get he’s just a baby still but it just seems really excessive. he always seems so spaced out too. Like there’s not a thought behind his eyes. Just wandering around aimlessly all day long. He laughs sometimes and screaches constantly but no talking

He started doing facial expressions about 2 months ago but he doesn’t look when you point at things and he can’t copy you