Hi, in a similar position here.. my son turned 3 last month and I have had suspicions on and off for probably 1.5 years.. mainly due to speech delay and social interaction, so some of your concerns resonate with mine.
First word at 18 months and only really over the last 6 months has his speech really progressed - he is a lovely chatterbox at home now, but won't speak to people outside me, his Dad and grandparents. His aunts/uncles he will become chatty with after a while, but if we meet friends, see someone we know in the street, meet someone new, and even at nursery - he won't or can't speak and will remain expressionless too. People always say 'oh he's so shy' but I know it's more than that, he is not hiding behind me embarrassed and shy, it is as though he just shuts down. He actively moves away from peers.
His 'obsession' is cars/vehicles, he would happily spend all day lining them up (and is very unhappy if you move one out of position), moving them round and round on the table or watching closely as he moves one back and forth. Over the past year or so he has had other obsessions alongside also - flags, house for sale signs, and now tow bars on cars.
He will constantly hold between 1 and 5 toys or objects in his left hand, meaning he never uses both hands for an activity. At one point it became a hygiene issue as he literally didn't open his hand for days.
He is a good eater in terms of amount, as long as it is the same (limited) foods on repeat. He can be funny eating in front of others.
He is funny about wearing new clothes. Won't wear clothes with holes in. He also will sometimes refuse to take his jumper off even though you can see he's hot.
He is the sweetest, most fun and loving boy, but I'm so worried he is going to go through life misunderstood. His Dad gets a bit defensive if I bring up my concerns, and my Mum always justifies everything, not really believing in all the 'labels'. I think other family members have had similar thoughts. He is under SALT and has recently changed nurseries (which is going better than the previous one) so I plan to speak to the manager next week about it all, and SALT whenever they follow us up.
Hopefully our boys are just finding their ways and developing their own unique and beautiful personalities and characters, but as Mums we can't help but worry, so best to speak up and explore our concerns if we can. Good luck xx