We had a similar case once where we had a mechanic quit to work somewhere else for better pay. Kinda left us in a lurch as there were some jobs he was the go-to guy on and we had to figure things out ourselves. The new place ended up working him much harder than he would like so he asked to come back. Then a couple years later the same thing happened (with the same shop he switched to prior if I remember) and he left us in a lurch again having to pick up the pieces. This time we changed our shop practices and made sure any time we as individuals were learning something, it was documented well and we as management were more able to fill in. A year or so later he asked for his old job back and we turned him down.
Looking back at his job performance, he was pretty cocky. He knew there were tasks we sent his way because it was his "thing" and he felt he was irreplaceable, and until we had the necessity to do it ourselves, we thought he was too. There were a number of jobs I myself took over after he left that I found to be a lot simpler than we previously thought. Technical, yes, but nothing a competent worker couldn't pick-up. It was a learning experience for us to maintain company information despite who has it in their head. Employees and Employers need to be on the same page as far as their compensation and duties are concerned, but in my experience if someone wants to move on because they don't like their compensation or duties, you have probably missed the opportunity to rectify things. I've also gained the opinion that I would rather fight to keep someone on payroll because they were easy to work with, quick learning, and reliable, then if they're only there because for better or worse, "they're the only one." Like a marriage or friendship, it's not healthy.
There's always the idea of MAKING yourself irreplaceable, but IMO it's just better that you could be replaced, but noone would ever want to.
Sometimes people just need to try something new and it doesn't work out. If he does what's needed and you have your companies way of doing things under control, him coming and going shouldn't be an issue, provided he works with you while you find replacements. If he's not reliable though, he's not reliable.