He can be immature andI agree with pp's that suggested a codependency.
A reason to move on. He will be a shockingly inept and emotionally harmful parent. Is that what you wanted for any children you may have?
They are enmeshed.
Enmeshed is emotionally unhealthy and YOU will be the target / scapegoat and will personally bare the brunt. Don’t put yourself in that position. A reason to move on.
One of his sisters is married to a man who is independent and he is not liked as it is thought that he is rude , when he simply likes to do things on his own and not get involved in all Organised activities when the family are together.
This demonstrates how irrational and abusive enmeshed families are. Blaming this man for behaving is a v normal adult way and the expecting his time to be dictated to by “the family” - they are all colluding in this back stabbing and this is v controlling behaviour. A reason to move on.
I have thought that he is not ready to leave his family emotionally.
You are correct. He has told you and shown you repeatedly that he has no intention of emotionally developing. Listen to him. A reason to move on.
He has said that if I treated his family the way his bil treats his family( which is not bad, he is polite but he is independent) that he would break up with me.
This is a threat. It is controlling and irrational behaviour. A reason to move on.
He is spoilt and the youngest and enjoys his ranking as the baby. There is ego stroking.
Yuck. That’s your job now as well to keep the brat happy. He is not capable of being an equal or supportive partner. A reason to move on.
Do these types ever grow up and fly the nest?
Nope. Maybe physically but never emotionally. A reason to move on.
No significant relationships in his past.
No surprise there - they all ran for the hills.