I wouldn't be force feeding yet. It hasn't been that long since his neuter. I would let him rest and continue to offer leafy greens/herbs. As long as he continues to eat a little bit of those every hour or two, I wouldn't worry too much about syringe feeding as that is just going to further stress him. If he wasn't eating anything at all after it's been 12 hours since surgery, then I would consider syringe feeding.