I think there is something wrong with my nephew

My nephew is 4, will be 5 in June. He’s always been an odd child to me. Unlike other children I’ve met. Very unlike my own children. As an infant he never cried. He was asleep 23/24 hours a day and never woke to cry or to eat. My SIL had to force him to wake up. As he grew up he was pretty much the same. Never cried as a toddler. Never really “got into things” like toddlers do, instead he would sit in any place you put him. He never seemed interested in much. He would play with whatever you gave him but not move. Not get bored. Not cry. As a child he’s still like this. He never complains or cries. He just sits there. His speech is very hard to understand. He talks more like a toddler would. I can pick out a few words here and there. But he doesn’t seem unintelligent or mentally impaired. He can write his name/letters, recognize all colors, shapes, etc. with what is appropriate for his age group. But he genuinely seems like he has no personality. He just exists. He never seems upset or shows emotion, but he will smile. Never in his life has he had a temper tantrum. I don’t think he’s ever expressed anger. Once, him and my son were playing with blocks, and my son built a tall tower. My 2 year old came over and bumped into it knocking it down. My son was like “aw no! I’m so sad!” and one of the blocks legit hit my nephew right in the face. He had no reaction. Just sat there. On top of this behavior, he barely eats. He will eat bread, cheese, fruit snacks, and French fries. That’s it. His constipation is so bad that sometimes he suffers from a partial rectal prolapse. But even with that, it’s no reaction. He just tells my brother or SIL and they shove it back in. I keep trying to talk to my brother that he really needs to be evaluated by someone. That something is not right. I don’t know what, but there’s something missing. My SIL is ugly about it, and tells me I’m jealous that she has a “well behaved child” and my kid is bad because he has ADHD. I am not trying to be mean or judgmental about it, but truly feel concern for my nephew. They don’t want to put him in school, they want to homeschool, so I fear he is never going to receive any sort of help. My brother is in denial like his wife is. But I cannot express how odd it is to be around this child. It feels like a shell of a person. Or like a person without a spirit. He has no reaction or expression about anything. It’s truly kind of frightening. No, he never misbehaves… he just doesn’t do much of anything at all.