I'd echo everyone else here. That's a rough but wonderful age. The sooner you accept that your child is different and embrace parenting the individual child you have, the happier you all will be. It really is just making peace with the idea that you will be judged no matter what you do, so know that you are doing the best job that you can and that your child is lucky to have you.
At this point, your child is likely somewhere near 99th percentile for milestones. If he were an adult in the 99th percentile for height, finding clothes that fit would be a challenge, toilets too low, beds too short, etc. The world just wouldn't quite feel like a good fit. If your child mentally continues on this trajectory, he is going to need help feeling comfortable with who he is and like he fits. Having you and your SO for support will go a long way toward not feeling so alien.
You will make mistakes, but it is okay. Your child actually needs to see you make mistakes and learn from them. You will be amazed how much your child absorbs from being around you, not only the positives like reading, but also any issues or insecurities you might have... Consider distancing yourself from people (including peer moms) who are truly not positive experiences for your child.
As for toys, my son literally loved to play with trash - cardboard boxes, empty pasta boxes, paper towel rolls, water bottles... Amazing villages were built, imaginary planes were flown... Also balls, wooden trains, hot wheels cars, the magnadoodle, a magnifying glass...
As your child develops, pay attention to what drives him... I read an article that said over 80% of kids are driven by appreciation for praise and feeling accepted by authority figures and peers, with a very small minority driven by the need for independence, autonomy, self-fulfillment... This small minority could not care less if they are rewarded with certificates, sticker charts, teacher praise... The signs that my son fell in that second category were very apparent from the time he was tiny, ie absolutely no interest in a stroller, refused to hold hands as an early walker, etc even though he was super cuddly. If I had understood this earlier, this would have saved some heartache. (On the bright side, at 4 he is really independent and can be trusted to load/unload dishwasher and do the laundry, although I still stand by to supervise.)
If you have a kid like this, only sign up for group classes that let you try them first and don't be afraid to bail. I committed to too many group classes thinking that since my child had gotten a bit older, it would be different... nope. My son still reacts with disdain if he thinks someone is trying to get him to perform. This is the child who could do a perfect standing flip into a foam pit at age 2 for fun after observing older kids do them for 5 minutes, but fully refuse to walk a line on the floor as a pretend balance beam for a potential gymnastics teacher...
This one is hard for me to remember sometimes... Your child may seem much older for most of the time, but may on occasion act his age, or have an emotional outburst like a younger child. My son who is 4, sometimes acts like he is 10, sometimes like he is 6, sometimes like he is 4, and every once and a while acts like he is 2. It is okay. The maturity will come and skills will develop unevenly, often in bursts.
As I said, my son is only 4. He is an extreme extrovert and always has been. He has always tried to engage other kids and adults, but up until about 6 months ago, his dad and I were his favorite playmates. We didn't do anything wrong, he just prefers his friends now. He makes friends really easily now, in terms of friends just to play with, but hasn't found many real peers. There is 1, who I think must be PG. She is one in a million and the two of them play amazingly well together with birthdays less than a month apart. The depth of their imaginative play and vocabularies (& sheer speed of their discussions) are unbelievable. If you find a peer like this who also is a good social match, do whatever it takes to nurture a healthy friendship. Unfortunately her family travels often so we don't see them much. His other friends he enjoys are age 6 and up.
The biggest difference between me now and me when my son was your child's age is that I have come to accept that I have no idea what I'm doing and I no longer worry what others think. I am much more protective of my son and have a lot less faith in institutions and experts responsible for kids. It isn't that anyone else is out to get my son, it's that within their realm of experience, they don't really get that he can exist as a self-driven unique little person. At worst, they see his drive/milestones as a trigger of their own insecurities. I try to take it one stage at a time, doing my best to make informed decisions that are right for our family. (No one else knows what they''re doing when it comes to parenting either and our parents were just as lost when they had us...). The fact that you are worrying about this means that your child is in good hands.
Exercise and museums are lifesavers!
If you and your SO feel your family is complete, it is. You aren't depriving your child if they don't have siblings.
You have definitely found your tribe. This is a place where people truly are supportive and understand what you are going through. Welcome!
