You tagged this question assertiveness. I want to go a bit further than the other commenters: the main thing for you to learn in this situation is how to become comfortable telling a service provider you no longer require their services.
Essentially, you're "firing" your plumber. There's no point dancing around that fact. You have a right to use any plumber you choose, and your current guy wasn't up-to-scratch (hard to get hold of, as you put it), so you found someone else.
This is not a pleasant thing to tell someone, so it's natural you're worried about annoying them. However, being overly concerned with being nice in situations like this might cause you pain in future.
You can remain perfectly pleasant and civil when you tell him you've found another guy. There's never any benefit to getting aggressive, and, as you say, you might need to use him again in future.
However, your mindset should not be "how do I show this guy that I'm considerate of his feelings?" or "Am I sure I'm in the right? Maybe I should have given him more time, after all he's a busy guy". In a personal relationship, this might be thoughtful and considerate behaviour — in a business relationship, this kind of attitude will lead to someone taking advantage of you sooner or later. (Maybe not by this guy, but by another service provider, or someone you deal with in the course of your employment.)
A better mindset is: "I've made the rational and justified decision to use someone who provides a better overall service. The most helpful thing I can do for the old guy is to tell him why I'm switching — I needed someone who was available and responsive, and my new guy does better on those particular things."
For example, you could reply to his text:
Thanks for getting in touch. Unfortunately, I've hired someone else to do the job this year. I would have hired you again but I couldn't get hold of you.
As I said above, what you say to him doesn't really matter (he'll have had this happen 100s of times), what's more important is getting comfortable being direct in these situations.