Oh Lord Billy, this is not your fault, never believe or accept that it has anything to do with you and it certainly has nothing to do with you doing something as normal as booking a holiday. It sounds so like he may be depressed, but you say he hasnt been diagnosed at all. Besides urging him to see his GP about something else and giving them the heads up in advance, I'm not sure what you can do to get a diagnosis, is he talking about any physical problems like back pain, headaches, stomach problems? Any other reason that you could get to refer him to your GP?
hugs,
Erin