Sending in Foreign Passport fore UK passport renewal

Just realised UK passport is expired for my son, for a simple renewal does he just send in his UK passport?

Yes.

So he can keep his US passport as we are intending to travel to the US in 2 weeks - I doubt the UK passport will be back by then.

Only checked his passport now when I cam to book tickets.

You can apply for a renewal using 1 week fast track or one day service.

Has section 4 been changed.

In the notes booklet it says

"If both parents were born on or after 1 january 1983...

On the form it states

"If either parent named below

My wife and myself are both UK citizens. She by naturalisation and myself by birth.

Parents nationality is not important when renewing the passport of a child. They are now citizens in their own rights. The section about parents and grandparents names can be ignored. They need the foreign passport to ascertain that the name in the 2 passports do not differ. All the rest is irrelevant as the form is the same for all passport applications.