A reason for it is that he is too lazy.
This sentence looks unnatural, because the whole structure seems to imply that there's only one reason for something marked by the word it. But yes, the use of a means there are multiple reasons. You just need to change the structure a bit.
The reason for it is that he is too lazy.
This indeed implies that there's only one reason. Or, that there could be additional reasons, but they pale in comparison with the reason.
You could even mention several reasons but then say "..but the reason for it is.." (with emphasis on the) to mark this reason as the most important and others, as negligible.
I will provide an example. Say, you want to retain the possibility of mentioning further reasons.
Imagine a teacher speaking about his pupil who failed to do his homework:
- One reason for it is that he was busy helping a neighbor.
- There's a reason for it: he was busy helping a neighbor.
Then he adds:
- Another reason for it is that his grandma got ill, and he was visiting her.
- There's a second reason for it: his grandma got ill, and he was visiting her.
Then he finishes his speech:
- The reason for it is that he is too lazy!
Here, he uses the and it's the key reason, from his standpoint. He thinks that it's the only reason, and there are no other reasons at all.
Despite first mentioning other reasons, he uses the to display his opinion towards these reasons. He suddenly lets us know that the first and the second reasons were provided by the pupil himself, as an excuse. But the third reason is the reason.
So it's a bit tricky.