Look at it this way... Yes, having a K-1 in the system shows increased immigrant intent. However, it also shows knowledge of the right way to do things and an understanding of the US immigration system. Make sure he travels with plenty of proof that he will be returning to the UK and he should be fine. Answer all questions 100% honestly and be prepared to show extra evidence of ties and plans to return to the UK at the end of the stay.
I made 2 trips to the US while my K-1 was processing and even though I had an envelope full of evidence I was never asked to show any of it. On both of those occasions I stated that I was visiting my Fiancee. In terms of evidence I had a copy of my K-1 NOA1, A letter from my employer stating they were expecting me back to work on such and such a date, bank statements showing I still had a decent sized balance in there etc..
There is nothing in any of the USCIS information which states that having a Fiancee visa in the system is grounds for denial of entry. As long as he is eligible to travel under the VWP and has no plans to stay in the US permanently this time around, he should be good to go.
Don't sweat it too much. You'll be fine !!
Mark