DS1 is 3.9YO and is not your average little boy (though lovely and very happy)
His speech was / is quite delayed (still often unintelligable)
He can't / won't hold a conversation he is not interested in
He has no concept of today / yesterday / later
he refers to himself in the third person constantly
He won't cooperate in any group setting (in all his life I have never managed to get him to a toddler class / playgroup / nursery without him doing whatever it took to get out of the situation)
he is scared of random noises (hand driers / hoovers etc) and covers his ears
We have had his hearing checked several times - never a problem, yet he often acts like he just doesn't hear you.
He has zero sense of danger and often actually 'goads' you - will run away, but if you run after him, he will run even faster... so you end up with a stand off where DS is too far away from to 'catch' him, but I know making a break for him will mean he runs in the opposite direction.
I could go on and on but I won't.
When I say he won't go to any groups:
- he will rattle doors if they are closed, to try and get out (disrupting group)
- He will just leave if doors are not closed (meaning I have to leave with him)
- He will not do anything he is told or follow direction
- He will scream / holler / shout if held in line / place
- He made himself sick when left at nursery (figured out that sickness meant getting sent home)
We have been asked to leave many groups, and not encouraged to come back to the others.
DH, DM and DF (DS's GPs) all still to refuse there is a problem 
He is very likely to get rejected from the prep school we had chosen for him and his brother which is very upsetting, though hardly surprising. I want to make an appt. with a psycologist but everyone else seems to think I'm causing this problem by keeping going on about it.
But, I've known for at least 18 months that things weren't quite right with DS1 and I don't feel it's helpful to leave it any longer.
How do you convince people / family that seeking to 'prove' there is a problem isn't a negative thing, but something that has to be done in order to move forward in the best way?