Hello and welcome! Do not feel badly about your feelings, we know you would not really leave him alone but I bet if most of us were honest, we would all say we fantasize about placing our difficult to parent kids out of the home or running away ourselves.
You will find that we all here have a perspective that comes from our own experiences so the things that jump out in your post will hit me in one way and others will share how it strikes them. We can't diagnose anything of course so you can sort through our responses and see what fits or at least see what would be worth checking through your own evaluations.
I am parent to a child on the autism spectrum. Many do not see it until they really know him, people who know autism see it right away of course, lol. I am also an Speech Language Pathologist (SLP) and (God plays funny tricks) I worked with kids with autism long before my son came along. Continued to do so (along with many other disabilities) for many years.
So that is my perspective. Your mentioning the obsessions with the toys, being an expert in dinosaurs, being very smart--already reading and working on other languages all can be signs of high functioning autism or Asperger's syndrome.
The other red flags that are common for kids on the autism spectrum are difficulty sleeping, hyperactivity/executive function issues (adhd-like), anger/rages??, and true, typical parenting methods do not work well. His being hypersensitive also fits.
(again, other things can account for this but I am sharing this perspective because I have seen it so much)
If a therapist or doctor does not work with kids who have the wide spectrum of abilities with autism they tend to not suggest it because they stick to the diagnosis only if they see lack of eye contact, NO imaginative play, no desire to be social, rocking, toe walking, hand flapping etc.... none of which, on their own, rules in or rules out autism.... but are symptoms that people most identify with autism.
Does your son play with those toys in a creative way, inventing new stories and creating new things for them to say to eachother? IF he plays with other kids with them could he create new play themes with his friends being able to contribute? Is he rigid in his play? OR does he tend to just share the same toys but play along side of others? Does he pick up on clues like facial expression and tone of voice the correct way or does he over react or under react???
how is his eating?? Is he restrictive in what he eats in any way? How does he do with sensory things, does he have clothing issues (tags, tight clothes, sock issues, certain color issues, etc...) Do sounds or smells bother him? OR does he maybe seek out activity, touching, smelling, tasting things?? How are his boundaries? Does he get in your space too much or does he not like anyone near him? Can he walk in a line at school (they have to be so close and stay in place)...
If these things hit close to home you may want a specific evaluation for autism at a child development center... if there are other possibilities (like a family history of any genetic conditions or mental health conditions etc...that can affect kids at any age) then this kind of center could still be good but a neuropsychological evaluation at a university clinic or a hospital based neuropsychologist center could really be a great idea.
Many of us also get Speech Language Pathologist (SLP) and Occupational Therapist (OT) evaluations (and since neuropsychologist takes a while, it is good to get these first and bring the results with you so that the neuropsychologist can consider these results along with the extensive testing they do).... Speech Language Pathologist (SLP) will look at social communication, expressive and receptive abilities including processing and can suggest if there are possible auditory processing problems. That is where his hearing is fine, but his brain may have trouble interpreting sound in the face of background noise or if two or more things are happening at one time, etc...there are many different kinds of auditory processing problems. You can imagine that if you had this problem, attention and judgement would look much like a child with ADHD. An Occupational Therapist (OT) can help you see if the symptoms are related to sensory integration disorder, motor problems (can be very subtle) and again can account for some of the hyperactive looking symptoms you are seeing. It changes how treatment is delivered, there are some specific therapies that can really help these conditions so it is great to rule them in or out.
Some books that many of us like are The Explosive Child by Ross Greene and What your Explosive Child is trying to Tell You by Doug Riley. For school issues Lost in School is also by Ross Greene and is wonderful.
Does he have an IEP in school? (special education plan that is individualized for his behavioral and learning needs). Even if a child is able to do well academically, especially at a young age.... he may need support so that he does not eventually be labeled inappropriately as a trouble maker, rude, etc.... It is better to have programs developed that allow for teaching of positive behaviors to replace the negative ones. The methods used too should be positive (mandated by federal law to be taught that way by the way....) so rather than missing recess or being suspended, he gets rewards and privileges for doing more and more of the skills they are teaching. Most importantly, an IEP gives you specific rights for him to have accommodations and adaptations if needed, like being able to take a break when stressed, having individual direct teaching if needed, having sensory accommodations like ear phones or plugs if things are too noisy, using visuals to support his learning (task boards, check lists, cues to help avoid misunderstandings). It all depends on what they find are his strengths and weaknesses in the school evaluation.
Let us know if this sounds like something you want to pursue, many of us have done it and can support you in your efforts.
Sounds like he is a great kid but really overwhelmed....something is interfering with his getting along right now. Hope you can find sources to do complete evaluations and that you can find some answers for his sake and for yours!