Does he have plenty of space (see the rwaf for minimum recommended, I'm not sure what it is these days!), and plenty of enrichment? Including somewhere to dig, hide, chew? I can't remember is he also bonded now so has a friend?
I had a rabbit who I'd hand reared after she was left at the vet practice I worked at the time, I already had a bonded pair and there was 4 in the litter (1 sadly passed away at 4 weeks) so once they'd had their vaccs I tried to re-home but found it difficult as often when I'd home check people either declined or things didn't turn out to meet the requirements, but I found a great home for 2 and by that point I'd had the female spayed and found I was too attached and couldn't find a home suitable so I kept her. I tried a trio which failed after a while as she was bullying my other female and her behaviour turned quite destructive, after being such a friendly rabbit previously, I separated her again and she became quite difficult, always lunging and biting, loss of litter training. I took her to the vets for tests as was sure she had a medical reason, spay tissue left behind causing the behaviour to be hormonal, but nope all fine. She was housed upstairs on the same floor as the bonded pair and I moved her downstairs as she was in our bedroom but she'd started jumping on the bed to bite us in the night etc, so she became free range in the kitchen/diner and overnight her behaviour went back to being friendly and loved to be around us!! So.long story short I think it was frustration being housed too close to other rabbits, difficult if you're in a flat but are the separated rabbits who aren't bonded together separate enough, ie no sight? Sound and smell cam affect them too, bigwig was separated by 2 doors and a hall but still got upset.
ETA: I find pet remedy spray works well and can be bought quite cheaply, it's herbal and suggested to be calming I find it does work
