You don't have to apply for the K3, but as they don't charge the fee now, you might as well and see what comes through first.
He can still visit the US on a waiver whilst things are pending, but.. he has to make sure he takes all paperwork with him regarding immigration, he must take anything to prove he still has ties to England, like work, mortgage or rent payments, anything basically that shows that he will or has to go back.
As long as Port of Entry believe that he is just here for a visit and will go back to finish the immigration process, he can be ok.
Often proving history of his past visits here is a good sign. If he has any old passports with date stamps on it for the US, good thing to take along too.
Some advice, as soon as he is asked to get a Police record report, tell him to do it as soon as possible as that can take some time.
Also tell him to make sure he is all up to date with all his immunizations including Polio. He needs records of dates etc of childhood illnesses too. If he can get the boosters at his doctor now, all the better. It will save time later and the medical will get through smoothly.
Check the medical list to see what is needed. It helps to know in advance and be prepared.
-----------------------------------------------------------------------------------------------
God Bless America - God Bless Immigrants - God Bless Poor Misguided Souls Too

National Domestic Violence Hotline:
1.800.799.SAFE (7233) 1.800.787.3224 (TTY)
Anonymous & Confidential Help 24/7