Pete - sounds like you have built up quite a bond with this fella. What you going to do, take less work on, or keep going and look for someone else? Where you based by the way?
Similar thing happen to me Worked with my son since he left school for nearly 10 years, got on great together and had a good working practice together. Built up a good client base witch required 2 men for most of the jobs inc commercial and industrial work. Then suddenly out of the blue he said he wanted a change of career which he has now done. Im unable to continue with a lot of the previous customers as there to much for a one man band and im not getting any younger. So it was either take on someone else, train them , have all the headaches of teaching them and then they move on again, so im now on my own doing mainly domestic work but even that more often than not requires 2 men, especially for rewires . Its now got to a point where i welcome small jobs such as the extra socket or light fitting changes