It depends on the executors - they would have to agree to your nephew moving in before probate was granted. Is there likely to be an IHT issue? If so, and there is not enough money in other assets to pay the tax, then he may be required to sell the hosue, or take a mortgage on it to cover the liability.
When my mother died last year her house (not contents) passed to her stepchildren based on her late husbands will,The executive solicitor told my brother and I to take any personal belongings and hand the keys over as soon as possible, long before probate,One of my caring stepsisters had the house clearance people in 3 days after my mum's funeral,