Hi My children have more than one passport and I have gone through this.....they can enter the UK but will be treated as a foriginer...my 3 month old was given a stamp that said...not permited to work!!!!. When you get his UK passport ( and I advise you to get it )use the Uk one to enter and leave the UK and his NZ to do the same here
I strongly advise any one who is entitled too, get their children registered with the UK consulate and GET A PASSPORT. I was born outside the UK and My parents didnt do this for me thinking all would be OK.. The goverment changed the law and I spent a long time fighting to get it..in the end ( 30 years! )they had to chnge the law but it was a long hard fight.