I would keep syringe feeding. It's important to keep their digestive tract moving so GI stasis doesn't set in. Though do be careful doing it if he struggles with accepting syringe feeds, as you just don't want him struggling and tearing any sutures.
https://wabbitwiki.com/wiki/Medicating_your_rabbit
If he's used to eating leafy greens/herbs, you could try some of those as well. I've had success getting my rabbits to eat leafy greens like cilantro, parsley, romaine or green leafy lettuces; following their surgery, when they won't eat any other foods(like hay and pellets) that first day.
https://wabbitwiki.com/wiki/Vegetables
Make sure your rabbit is staying warm. They will often get chilled following surgery, and a cold rabbit won't eat as well. I will usually feel their ears and if the ears feel really cold, that means their body is also probably cold. A rabbit safe warm pack should be used, making sure it isn't overly hot, before placing next to or under your rabbit.
If you can get meloxicam suspension, that's usually what's prescribed, at a dosage of 0.5-0.6mg/kg, twice a day for a few days. Aspirin shouldn't be given as it is a blood thinner and your rabbit just had surgery. And other otc human pain relievers aren't commonly recommended for use in rabbits.
If you still feel he needs a pain reliever and you can't get meloxicam suspension otc, then you may need to get him to the other vet to get the meloxicam suspension prescribed. Just make sure the vet prescribes the right dose, as rabbits need a higher dose than other animals. Rabbits usually need at least 3 times a dogs dose.
https://www.vgr1.com/metacam/
Just be aware though, that even though meloxicam will often help get them eating again because it gets the pain and inflammation under control, sometimes it won't help if they've stopped eating due to nausea from the anesthetic, and not because of the surgical pain. I had one rabbit with this problem. Took 2 days before he would eat normally again, while he was getting dialy high dose pain relief. But he would eat his leafy greens, so I just kept offering him those all day long, then he started eating his other foods by the second day.
