My 17 yo brother acts like a literal baby

My brother is 17 and he’s extremely affectionate with our parents, to the point where it honestly feels strange to me. He’ll go around the house demanding hugs and kisses, asks my mom for foot rubs, carries around her clothes and sniffs them because he likes how she smells, and sometimes still sleeps with our parents. He also cuddles up to my dad, sniffs him, kisses his face over and over, and snuggles into him like a cat. Even my dad has said it’s weird sometimes and that he needs to man up.

He also makes baby noises all the time and almost exclusively speaks in a baby voice when we are at home even when I have friends over which is embarrassing. He calls my mom mommy and my dad daddy like a kid would.

He’s otherwise a pretty typical teenager, so I’m just wondering if anyone has seen anything like this before. I know every family is different and I’m not implying anything inappropriate is happening, but this level of attachment at 17 seems unusual to me. I am pretty fed up because he does it to me too , asks for foot rubs and kisses and hugs whenever I pass by and calls me nicknames they used for me as a baby. He also didn’t just start doing this , he’s BEEN doing this ALL the time and my parents never did anything about it.