I might have problem with my 4yo DS's behaviour....

[confusedperson]

I am quite nervous on writing here, but here I am... I have 3y9months DS, who always has been difficult child, i.e. very demanding, short temper, lots of biting, defiant etc. He is really difficult to handle, so I started researching behavioural disorders, and matched it with Oppositional Defiant Disorder (ODD) - constant arguing, defying everything, intentionally bothering people, always pushing boundaries, easily annoyed, lots of tantrums, touches his brother's hair all the time, short temper, some anxieties like door has to be in a certain position. He also completely does not understand word "no" - if you say "no that's not acceptable" he would say "yes it is" and almost laugh in your face. I say "it is not funny" he says "yes it is funny".
Now I understand that lots of normal children have some of these traits, but I am having really hard time with my current nanny who is absolutely unable to handle him - a 4yo child!!! I guess she tried to be friendly in the beginning, and he took her as "no authority", and now he completely does not listen, does everything in opposite and started biting and pinching her when he does not get his way. My nanny complaints about his behaviour every day, and I feel she will quit if this continues. I explained her how to handle (no engaging in dialoges, clear reason-consequence, lots of praise), but still she cannot. I cannot blame her he is a disaster constantly making everyone's life miserable.
The feedback from 15 hour free nursery he goes is "he is pushing boundaries, but still within normal range. Occasionally bites or pushes other children. Very creative and bright. Should grow out but needs very firm boundaries".
I used to think he is on the edge of normal behaviour and behavioral disorder, but now as a normal adult is not able to handle him, I am afraid it is much more to that. I was going to wait until he starts school in 8 months time and see how he is doing there, was hoping he will grow out slowly, but he seems to be getting worse especially with our nanny. Our other DS2 (14 months old) is very happy with our nanny, though...
Any advice how to handle and whether I should be seeking a specialist help, would be really appreciate. I already got books How to handle Defiant Child and Playful parenting, but we still have plenty of issues which makes our life miserable.
Sorry for this being long!

It takes forever to get a dx of anything (normally at least a year), so I wouldn't wait 8 months if you're already finding everything really hard.

Go to your GP and ask for a referral to a child development paed.

Does he go to nursery? How do they find him?

I think OP mentioned nursery already Indigo.

Sounds really difficult and nanny is complicating things massively, so hard to see what is really going on.

agree with Indigo that I would not wait. If he has no obvious neurological markers - like speech delay or poor eye contact - it might be worth looking into private advice as you'll be waiting forever on the NHS and they'll just adopt a wait and see approach in all likelihood.

I have a son the same age with a known neurological condition which does involve some defiance, but he doesn't sound as difficult as your son!

Sorry, I didn't read the bit about nursery (More white space / paragraphs makes a post easier to read....)

If nursery say 'he's still within the normal range' that rings alarm bells for me.

There is no reason to assume that nursery staff know what the 'normal range' is. That's very different to saying 'we don't have problems with him'.

I'd def get him checked now. You have problems with him, your nanny has problems with him, and nursery have problems with him.

Being bright and creative absolutely no relation as to whether or not he has SN.

Nurseries and Recpetion staff often say 'he'll grow out of it' - but truth is they don't see the kids in 3 years time, so they don't even know which kids did and which kids didn't grow out of it. :(

Omg. That is why I was so anxious of posting here. Sorry for no gaps between the paragraphs, only noticed it after posting. The nursery said that he is within normal range because I asked them whether his behaviour is normal for kids of his age. He started speaking quite early and he is bilingual, is quite chatty, although sometimes does not make sense. He does make eye contact. He is active, but not hyper. I could not relate him with typical ASD symptoms (due to good speech development, quite social with somebody he knows, and no noticeable attention deficit or hyperactivity).
Obviously nanny does not do him any good (not that she is a bad person), but I cannot afford to put both of my DSs to nurseries, that?s the problem. I am thinking into what exactly I should be doing next?