What do you do when you approach the subject of noticing some changes in your partners behavior of being unemotional?

We've been together for almost 3 years and this is the first that this has happened majorly. I've confronted him about it but when I initiate these kind of talks (especially recently) about his sudden changes he just says he's fine. Then i instantly see him shutting down. We've never fought before or had any serious arguments. We did in the beginning, and when he noticed my change of behavior (family and work problems that I didn't know how to talk about at first), we addressed it. Together. Been great since. So, im at a loss. I noticed the change sometime in June. But got to thinking that it might've been in May I just didn't pick up on the changes until June. What spurred me to start questioning his behavior was because of him going to urgent care for a really bad head cold but I didn't know about this until a few days later. He didn't think it was that important to tell me at the time. We've got out very first vacation coming up at the end of the month and I dont want this hanging around us before or during our trip. Any kind of either advice or something to help me get him to open up without him shutting down.

Thank you in advance! And sorry for the long message 😅