Okay but I think that the mom's right and he misinterpreted it
The marriage advice is "if you let your heart waver, you'll lose time" and the romance prospect is "things won't work out as you expect."
Even though he's (probably) gay, homosexuality is still pretty uncommon, and even bisexuality is kinda rare, though less so.
So he may kinda know in the back of his mind that even if he confesses, he'll be rejected or at least that's his concern
The twist is that he won't be rejected and they'll like one another.