hereidrawtheline, you really don't have to try and convince anyone that the problem is not discipline. But it is totally natural to think that it is, somehow, something you have done. Especially until someone says "no, there is an issue here.." Because until that point you are just dangling, thinking why won't my child respond to things the way he/she should?
It does sound like he has some sensory issues, and certainly wants the world to revolve around him. Of course, that is normal to some extent, but if it is extreme, it causes a few problems.
My DD is 3.1 and has some sensory issues, as well as developmental delays, but is quite similar in some aspects to your DS in her behaviour.
We have had some things recommended by the Area Inclusion Coordinator for the Education Service, which have helped (as a result of her assessment, DD1 now gets full 1:1 at preschool).
-Motivator box: Try to work out what textures or colours, etc. he likes. Or just what makes him happy. Find objects in places like Hawkins Bizarre to put in a "special box". That box is kept separately from all his other toys, and out of his reach.
If he is having a meltdown or just a generally difficult spell, you can offer to get the box. Then he gets to play with the things in the box. But as soon as he loses interest or goes off, the box is put away again.
For our DD, her box is filled with things like 'noisy putty', a jelly caterpillar, a windmill, a rubiks cube, a gel ball, a stress ball, a water snake and a rigid bouncing ball with lots of different coloured bands that all link in together.
-Sometimes when children crave attention, any attention is good attention. Therefore, even when you are saying "No", it can be feeding that craving for attention. Avoid saying "No", and instead, find a positive instruction to give your child. So, for example, if he throws a toy brick, instead of saying "Don't throw bricks, DS", you should try to say something like:
"DS, I'd like you to put the bricks in the box now." This way, he is getting the attention for doing something positive, and you are achieving your goal, which is to stop him throwing bricks.
These things have given some improvement for DD.
Hang on in there! Whatever the cause of your DS's current behaviours, the key is that you don't blame yourself.
I think, also, that it is possible to be somewhere on the autistic spectrum even with emotional and expressive qualities.
Regardless, I hope you get some real practical help soon, because a diagnosis can take some time, and right now you need steps towards a solution