The six month expiration is for activation of the visa upon entering the USA. As long as your Fiance makes the move from the UK to the US within that six month window, he will have no problems. Whenever he arrives in the US and activates the visa the POE officer will scribble all over the visa making it void. The K-1 is a single entry visa meaning that once it is activated, the 90 day count starts from that day.
To clarify... In your situation, with your Fiance planning to arrive with a month left of that 6 month activation deadline, that won't give you a month before the 90 day count starts. The day that he arrives in the US starts the 90 day count.
As a side note the deadline for you to be married is the same date as the expiration date that will be stamped on your Fiances I-94 (90 days from the date of entry into the US).
Hope I haven't confused you there !!
Mark