His nomination has been formally submitted to the EU Council last week (PDF incl. his official CV). Cameron, who had his last day in office today, wrote in the letter:
"Sir Julian is a very experienced diplomat with particular expertise in European affairs who has served successive British Governments with distinction. I believe he would be a strong addition to the Commission for the period ahead."This followed the official resignation of the current Commissioner Jonathan Hill with effect of 15 July 2016, midnight (PDF with Juncker's letter announcing the resignation).
As I wrote previously, it will be interesting to see what portfolio King gets and what kind of games the European Parliament might be willing (or not willing) to play ahead of his potential nomination.