Fear of losing the connection to what he values.

Before he sleeps, do this with him and tell him why:

Look at me, into my eyes. Words may or may not be needed. Feel the feelings and the fears when we look at each other. Try to allow those to show. Let the smiles happen, let the shyness. Don't hide from each other what you feel might be show, try to release walls that may come up. Feel feelings while you look. Say things to reassure. "I am here. You will not lose me. We are safe. We have made certain nothing outside can get in and harm us when we are vulnerable".

Etcetera. If his spirit feels safe as he goes to sleep, it may help.

Only time can truly heal deep seated fears. Never repression, but always understanding, growth and eventually - wisdom.

At least he is looking for you and not running away. Marriage is a big step, you are engaged and this will put pressure on core ideas - knowingly or not.

Marriage means: I will become more, I will owe, I will be owed, I will do, I will fight, I will give, I will demand, what do they need, what do they want, can I become, and so on and so on.... It is deeply, personally demanding to the psyche and not to be entered into lightly.