Hi OP. I understand what you are saying! Here is my experience: I first noticed mine was avoiding close eye contact around 6m, but he was otherwise developing normally.
He was always very flappy and hyperactive and would run in circles for ages. Around 2yo the differences between him and other kids started to become more obvious - he wasn't listening to me at all. He didn't play toys in the way others did. He flipped through offers of books at great speed but had no interest in me reading to him. He was obsessed with pouring sand and water. He was falling behind his communication milestones too, but his dad and the HV dismissed my concerns.
At 2.5y, he seemed to regress. I would call him and he wouldn't respond at all - that really triggered the worries in me. I asked on Mumsnet and everyone said autism. He started OT and ST and he made progress, though hard to say if it was the therapy or just him growing up. Funnily enough, he started making lots of eye contact again during lockdown. I think lots of time with him at home helped. Eye contact is not an issue now.
Now at 5y, he's not been assessed but there's no question he's delayed and is probably autistic with sensory processing disorder. The SPD was blocking him from learning, with therapy now he is in a preschool and learning lots. I am often surprised that things I was saying 2-3y ago have suddenly popped into his vocabulary. He was listening back then after all. His teachers are confident that in time he might manage in mainstream school. I think I'm most concerned about his social skills - he's longing to play with other kids but doesn't know how. Lots of things are hard. He can get very frustrated and physical. He's otherwise pretty happy.
I think for you, the lack of interest in other kids is not unusual at that age, but I can understand why you are concerned. It's too early for any kind of assessment, so I would just advocate trying lots of different toys and ways of playing to see see what engages him, and follow his interest. Don't worry that he's not playing with toys in the way you expect them too, just make a note so you can keep track of his development. The post you made is great - save it.
Also, maybe look at Oxford Baby Lab. They are a centre that is doing research into babies and young kids and communication. I remember reading a study about how they are teaching parents strategies with potentially autistic kids, younger than the age they are normally assessed, that may help address some of the difficulties. I think they teach parents how to interact effectively with them. Too early to know if there is any long-term benefit, but I wish I could have got my DS involved in the study when he was 1yo.
It's also too early to look at Hanen books but look up More than Words which is about strategies on effectively communicating with your kid through play. If you continue to have concerns, it can be helpful from 1.5-2y and older - it's just a really expensive book.
Hugs - I know it's hard! Many will say it's too early and that these things are normal. So I would work on looking at ways to keep your child engaged with you and going with his interests. And you can start voicing your concerns with your HV or GP and what support there is down the line.