Sorry to hear you're going through this op.
First thing to do is make no drastic changes or decisions.
Mid life can throw up all kinds of things feelings about life slipping through your fingers.
People who have only had one adult relationship from a young age can find they've changed but the relationship hasn't, and/or that you wonder what else life has to offer which is often connected to that sense of your potential being more behind than in front.
Sometimes this will shake and break your relationship.
Sometimes the person works through it and finds the source of the issue was not the relationship, but they needed to acknowledge they were in a new life era which throws up questions about who you are and how you have/are spending your life, make some changes, inject some fresh thinking to break out of the ruts... and if the relationship can change and adapt then the person finds they don't want to throw the baby out with the bath water after all.
When you are sharing life with someone each others turmoils are shared turmoils, so you end up going through it all together. Even when your own internal emotional landscape is (was) calm and happy you have to face the challenge anyway.
You and he both need to recognise that his feelings could be about his relationship but could be about life in general.
It will hurt, and that's got to be faced because otherwise it will be swept under the carpet and choices will be made based on avoidance not working through it.
You also need to be wary that how you support him as he works through this isn't at too great a cost, so you don't sell yourself short when agreeing supportive actions.
So what you need to do is say...
"I hear you are fundamentally feeling unhappy, that hurts and was a shock.
I think this needs to be faced and taken seriously, but I also feel it is fair that you get help working out what is at the root of it so you don't sacrifice what we've built together until you are sure that's the right thing to do.
I want you to talk to friends/family/counsellor and really look into yourself too understand if this is a life thing or an us thing, or both.
While you do that we put a pause on any changes until you can honestly say you've done all you can to understand your own wishes for the future.
It is fair if you are going to drop this bombshell that you do this so at least, given I don't want this, if we go out separate ways I know it was the only way."
Your shock and distress is palpable and this is awful for you, but better you face this than you just allow fear of his inner feelings to create a situation where all is well on the surface but he's going through the motions.
Because that will mean you both drift into a situation (whatever that may be).
Grip the steering wheel of your life.
He finds guidance and clarity somehow, does the time self examining and then whatever the outcome is the right one.