If he's losing weight and won't eat the new pellets, I'd be inclined to just feed the old pellets for now, and feed more (enough that it just runs out at the end of the day and it's time to feed again) until he settles in better and his weight isn't such a big concern. I know it's crappy food, but a new bunny not eating well and feeling skinny, is more of a concern to me.
Then if he's doing well with this increase of feed, and is starting to fill out because he's eating and drinking better, then I'd very gradually start adding in some of the new pellets, while decreasing the old. Make it very gradual over several weeks, so than he has plenty of time to get used to the new pellets little by little.
If you want to very gradually introduce some leafy greens like parsley, cilantro, etc, that's up to you. It may be too soon though. Some people prefer waiting until 12 weeks old before introducing veg and greens. But whenever you do start, it's best to only introduce one at a time, and very gradually over a week at least, starting with a small amount and only increasing if there are no signs of mushy poop or digestive upset.
https://rabbitsindoors.weebly.com/greens--veggies.html
I would suggest weighing him every day to make sure he's starting to put on weight with this increase of his old pellets. Plus always have some good grass hay available, maybe a little alfalfa, and hopefully as he settles in he'll get curious and try eating it.
There is also considering the possibility that the appetite issues and weight loss, are due to a health issue. I know of someone that had a runt, that it ended up being a parasite issue. Once the bunny was dewormed, it started having normal weight and growth afterwards. And a runt I had, went on to fill out and grow to a normal size after she was put on a higher fiber diet and better quality food than she got in her previous home. But I didn't have any problems with her being picky about changing foods (gradually). There's also the possibility of a dental issue that is the cause of your rabbit not wanting to eat hay.
So it may be your bunny needs to be seen by a knowledgeable rabbit vet. He may even need to start having regular syringe feeds if you can't get his eating and weight stabilized. You just don't want a lack of adequate eating and food consumption, to result in GI stasis. So seeing a vet is definitely something to consider if he doesn't immediately start eating more of his old food and start putting on weight. Or if after a while and trying different things, he still won't eat hay.
https://rabbit.org/care/gi-stasis/
https://wabbitwiki.com/wiki/Veterinary_emergencies
https://wabbitwiki.com/wiki/Choosing_a_rabbit_veterinarian
https://rabbitwelfare.co.uk/recommended-rabbit-friendly-vets/