How old are the children? Your partner can apply to the court for an Order of Division and Sale but if his ex can show that that would be bad for the children he may not be successful. The courts are very keen than children of divorcing parents do not suffer unnecessarily and a stable home is considered important, especially if they are at school and there is not a suitable smaller area in the vicinity.
If your partner is able to provide a home for himself whilst his family continue to occupy the marital home then it is unlikely the Order would be granted. There will be no problem when the children are grown up.
In any case, downsizing in the current market could be a very bad move, financially. Larger houses have lost more money than smaller ones.