Responses
I think it's just his mindset. Usually you think of a proposal and you are filled with happy, joyful feelings but he's feeling insecure, anxious and is afraid of losing his partner. I don't think he wants to take such a big step under the pretense of just putting his mind at ease because "if I propose he'll never leave me" is kind of where his mind is and he knows that's not healthy.
After reading all that i still dont understand why hes sp hesitant about proposing to him cause hes scared he might lose the hapoy relationship now? Is that it? Im a pretty simple minded person and hes the type to overthink a lot and i believe his partner would feel the same happy guy if he proposed