I don’t think he is any less happy than when you first adopted him. He probably is just getting more comfortable with you and your home.
Sometimes when I open up a new area of the house to my bunnies, they get excited and start binkying. After a few minutes they calm down-not that they are less happy, but I suppose the initial excitement has worn off? Binkies aren’t necessarily the only sign of happiness, either. You are giving him a great life and he knows it.
Changing up the “landscaping” can encourage them to explore and play. Moving tunnels, boxes, and other playthings or hiding treats around is a great way to engage your bun.

I agree with LongEaredLions. He may just be settling in still. My rabbit has days where he is perky and playful and days where he wants nothing to do with me, happy to play on his own. I even get nudged to go away.

Just keep spending time with him, especially sitting on the floor when he’s out. Curiosity gets the best of them and he’ll come check you out and maybe settle in for pets. It took my bunny about three months to realize we were going to be true buddies. He was always friendly but it took that long for him to start hanging out with me in a routine way. Cut up some fresh cardboard or toilet paper rolls to leave around as surprise treats. Rabbits can’t help themselves, always needing to check out what is new.

All rabbits are hard to read so I understand why you are worried. But time and dedication will make it easier. I started training Olaf to do tricks and he is now really excited when I get the treats out to play. It is a fun way to bond if you are interested in that sort of thing and can be practical if you teach them things that will make both of your lives easier (getting into the cage, carrier, etc.) Good luck!

The first few months were weird for me too! If he was at a shelter, you may have no idea what kind of life he had before, so you don’t know what he’s expecting when you move or go somewhere. I got Demi directly from the home that neglected her (Craigslist), so I had an idea of what she’d been dealing with. I guess it was easier for me to be patient because I had that information. I can tell you that you’re learning about Gordie almost as much as he is learning about you! Just be patient and consistently loving, feed him on schedule and wait. It will happen!