I need help

Literally so confused. My son who is 18 months has no words. He sings to himself (songs I recognise like abc or wheels on the bus). He can sing a specific part of the song “up and down” and puts his hands up and down and ABC he sings the first few letters very clearly. But other than that, he has no words. He doesn’t play with toys just loves banging them together or banging them on the walls and tables. He has no hyperfixations like spinning toys but definitely loves banging things together. He says dada but has never said mum or mummy or mama. He understands stop and no and give kisses and come here but nothing more. He doesn’t point. Babbles all day non stop to himself. Sleeps well and is sleep trained. Eats well too. HV recommended doing an autism application for an assessment. We were rejected on the basis that there wasn’t enough evidence. Everyone I’ve spoken to like nursery have told me to give him more time but something doesn’t seem right and I can’t put my finger on it. He doesn’t have typical autistic traits so I’m not sure if it’s a developmental delay or if I should push for the autism assessment. We were also rejected for speech and language based on lack of evidence again. I’m losing my mind because he seems to have no interest in learning anything new but wants to just do his own thing. He can walk and run really well and I’ve seen posts on here saying that kids sometimes focus on one thing at a time and I’m wondering if he’s more focus on moving around and running than learning words. He also has no real interest in anyone but me or his dad. No interest in other kids. He gives good eye contact when he wants. Responds to his name when he wants. Understands if I tell him off for something naughty. I’m just so confused and could do with some advice or insight. I’ve never really been around kids before I had my own. Please please any advice or opinions are welcomed. I just need answers about what’s going on with him.