WaftyCrank, I am going to tell you my story in the hope that it will help you, but I don't want to worry you, because I am not saying that your DS is the same as my DD 
Are you sitting comfortably? 
Then I shall begin....
DD1 screamed all night every night for her first 12 weeks of life. Slept all day. Then, she suddenly settled at 12 weeks into a routine of sorts, although she woke several times per night.
DD1 was a beautiful, lovely little girl. She was (we thought) bright, inquisitive and paid attention to detail. She was hard work, but then often bright children are. The things that we thought signalled bright are quite laughable now. But, she just didn't seem to go through the irritating stages that other children did - she wasn't at all bothered by us leaving the room. We went to church, and we could leave her in the creche quite happily while all the other Mum's struggled with separation anxiety.
At around 8 months, she had her 8 month check and I commented that she didn't seem to respond to sounds in the way I expected. She didn't seem bothered by sounds, and didn't respond to much to a door closing, although it was clear that she could hear it. She also didn't seem to be babbling as much as other children her age. The HV noted our concern but 2 months later, she had picked up another sound, so the HV declared her normal.
DD1 didn't find it easy to pull herself up to standing, and by 15 months I thought her posture was odd. HV brushed it off, but I went back, and at 17 months we saw a physio. She thought DD1 was 'normal but a bit slow'. At 20 months, DD1 was cruising (just) and she almost gave us piedro boots, but decided against it. DD1 walked finally and haphazardly but determindly at 23 months. So she was declared normal.
Roll on several months. We had got used to the now mobile DD1. She didn't seem to understand 'no' or boundaries. She just laughed, and would 'smack' her own bottom, grinning. No discipline worked at all. But somehow, it was clear that she wasn't actually being defiant, just that she didn't 'get' naughtiness. She would stare intently, and people saw it as a sign of intelligence - 'taking it all in'.
We got used to DD1 in our own way, and modified our way of being. Stairgates everywhere - we even used to take a travel stairgates 100 yards up the road with us when visiting our friend who had a staircase which opened into the lounge. It was the only way we could have a cuppa without spending the entire time retrieving DD1 from the bathroom. Looking back, it is crazy that I thought DD1 was 'normal' but I was a 1st time Mum, and nobody says anything, do they?
At 2.6 DD1 started preschool. The preschool manager commented on her gait being 'immature'. I said, "well yes, she did only start walking 7 months ago". I told them that they shouldn't leave the door open because DD1 would escape. They said "We do know what we're doing...". Two weeks later, her keyworker said "I can see what you mean!".
Anyhow, she seemed to settle fine (no separation anxiety don't forget) and at the end of term, I thanked the preschool leader for welcoming her in. She said "I'm really looking forward to seeing how DD1 develops over the Summer, to see the change in her in September". I was oblivious and thought 'ahh, how nice ' 
DD1 returned in September, and within 2 weeks, the manager and the SENCO approached me to say that DD1 was 'a little behind' and they felt that she could do with some extra attention, so could they refer her to the area Inco? Something in me clicked. I just knew in my knower, as they say, and deep down, I knew that they were right, and that DD1 wasn't quite 'right'.
My sister was FURIOUS when I told her. She said "HOW DARE THEY SAY THERE IS SOMETHING WRONG WITH MY NIECE". I replied that she was my daughter, and that I agreed with them, that there was something not quite right.
The area Inco saw DD1, and agreed that she needed 1:1, so applied for funding which would take some time to sort, and preschool started giving her 1:1 straight away, saying that they would sort out the funding issues later. At this point, the Inco still felt that DD1 was just 'immature' rather than having any sort of condition, although she agreed to refer to a paediatrician because I raised concern about some sensory features (always putting things in mouth, scared of adverts, some cartoons, some noises, had to touch certain things, had to do roly-polys on the grass at certain points).
Fortunately, for us, DD1 started falling over for no reason at preschool. They kept telling me that they weren't sure what happened, they couldn't see what she had fallen over, or why she had fallen. I took her to the GP, who diagnosed a virus. A few weeks later, she fell over at the playdough table. She was stood playing with playdough then pole-axed to the floor. No warning, just standing then led on the floor. I walked her home, and she fell out of my hand-hold twice on the way home. I took her to the GP, who sent us to the hospital. They realised she was developmentally delayed, kept us overnight, and arranged ECG, EEG, CT Scan, etc. The EEG showed Epilepsy but the CT was clear.
A few months later DD1 had an MRI scan, which showed her brain abnormality.
Now, 2 years later, it is clear that DD1 has learning difficulties, sensory issues, etc. She starts Special school next week.
I don't want to worry you, but the evidence shows that the very best thing for children with any special needs is EARLY INTERVENTION.
Take this opportunity and RUN WITH IT. If your DS is just a little 'quirky' or 'immature', the paed will send you on your way. If not, you will be grateful that he is not starting school with teachers thinking he is awkward or stubborn or oversensitive. They will be able to meet his needs.
Let us know how you get on. Feel free to lurk or jump in. Ask questions, whatever.