It would be very important to find out why he has a GI stasis, but also right now, he really needs enough liquids and food to keep his tummy and body going.
I have had to give a lot of Critical Care to bunnies over the years, once in the case of a tooth root abscess while the bunny was healing post-op for several weeks. And the bunny did not want it and I was desperate, so I know how you feel. But somehow I figured it out in time. Still hard though! Something that really helped me was reading that *it's not usually the food itself the bunny hates, it's the stress of the situation how it's been given*. So while it's called "force feeding", it really, really helps to be as gentle as possible and not do it by force. Patience and trying different methods until something kind of works.
First of all, do you have a big feeding syringe with a long tip? Mix a teaspoon of critical care in a mug or cardboard cup with about 3 teaspoons of water and *spoon it into the syringe through the bigger end*. Then assemble (and push the air out the top). It's good to make the mixture a little runny rather than too thick because the bunny needs the water too and it's a lot more easier for him to eat and swallow. But if it's too watery, it won't give him a lot of nutrition.
Every bunny is different, but what has worked for my bunnies really well in giving medicine and food is by setting the bunny on top of a towel or blanket in my big office chair (it has arm rests so feels a bit like a big nest), or you could try an armchair. Sofa offers too many escapes. Then I sit in front of the bunny on a little stool or on the floor and I have great view and access to their little mouth and see what I am doing! My bunnies don't try to leap off the chair and I do not need to restrain them much during the process. My bunnies actually love the taste of critical care, taking it even voluntarily as a treat, but only when they are well. When ill, they do not want it.
So, I would usually have the bunny facing me, and then kind of corral their head gently with one hand and try to insert the food with the other. Only give about 0,5-1,ml at a time, and start with a tiny amount. With the long tip of the syringe, go in through the gap behind the front teeth and don't aim straight at the throat and also not straight to the the other side, but diagonally towards the opposite cheek. Be gentle and stroke the bunny and give them time to chew and swallow between mouthfuls. The bunny may resist, most do. Some bunnies throw their head up or turn around and some may tuck their chin down to their chest. Calm them and find the tooth gap and try to gently get some food in. You can ask another person to hold the bunny a little, but I find often this just makes the situation feel more stressful for the bunny and manage better on my own. Wrapping the bunny in a towel might help too, but I've not tried it. One of my bunnies takes the food best when I sit him with his left side snug against the back of the chair, and the feeling of this "wall" kind of holds him in place, and then I aim through the gap as usual. There comes a point in feeding when the bunny just wants no more food, and I will not fight beyond that point, but you do need to get something in every 3-4 hours if they aren't eating.
As for extra fluids, a lot of bunnies won't take plain water from a syringe and water is really hard to give if they don't want it (always be careful not to get it into the lungs), but if you mix it with a little unsugared baby food apple mash for flavour, I find buns will often lick it from a syringe. Sugar is not good, but this contains such a small amount and fluids are really important so the bowel contents or the bun won't "dry out."
Write down how much food and liquids your bunny is getting and at what times, and also keep an eye on his poop production and hopefully things will improve. Best wishes for your bunny.
