Ahhh, living with bunnies can be highly rewarding (but sometimes a little complicated, especially when they’re all singles lol).
Once your Holland boy is all “aired out” from his neuter, you could let him be neighbors with the girl bun. Even being neighbors can give them a sense of companionship, I think, and may help him out. They may even “hang out” eventually (in their respective enclosures) or along the boundaries. 🙂
I suppose another option would be to get your Flemmie boy neutered as well and attempt to bond him with the Lop boy, although you would have to be vigilant during the process due to the size difference. I have a pair of bonded boy buns and they’re the best of friends. And years ago, I had a Flemmie girl bonded with a normal sized boy bun (although, to be fair, she was only about 13lb so on the smaller side). But, I could be mistaken about this option, so I would see what others suggest maybe.
I’m not sure where you’re located, but if you possibly have a House Rabbit Society chapter in your area (or State, if you happen to be in the US), they may have a list of low cost spay/neuter clinics that accept bunnies in your area.