Stop doing things that haven’t worked. More of the same will yield more disappointment and frustration.
I’m not sure “Irrational’s” behavior is irrational to him. He’s behaving in ways that make sense to him.
The fact that his first year went well indicates he knows how to perform within your culture.
Clarify what your team member wants for himself, colleagues, and your organization. You may believe you know what he wants, but it’s worth a conversation or two.
How do behaviors reflect and align with wants?