What was your boys diagnosis? It sounds like gut stasis but would not want to assume.
I have had a real fight with gut stasis this year, it can be/is a really tough thing to work through so please be kind to yourself in this time.
As for syringe feeding, please give yourself and him a chance, my boy absolutely despised getting syringe fed and would battle me for a good few days and then suddenly he was fine with it. It even became his favourite part of his day, as he was being fed almost every 2 hours for a few weeks, he would let me know when he was ready for more by putting his mouth on the syringe.
Another thing you can try is feeding it to him via a spoon/bowl, but if he is displeased with the fact he has to eat it in the first place this may not work.
If you are really struggling with it, and your boy enjoys nuggets, you could try adding some pellet mush into his critical care. Or if your boy is losing weight and you are still struggling with administering critical care you can feed pellet mush alongside critical care. There is a bit about such in the guides linked above.
Make a routine for you and your boy, if you follow a loose timetable you may find the stress to wear of a bit.
If your boy is still eating hay, as mine was, I kept him on 30-40ml of critical care a day as he was eating hay on his own. Around 100ml a day is near the total calories a piggy will eat in a day, so you may find this helpful to keep in the back of your mind. Weighing daily will/can help you decide how much your boy needs in a day, as it will depend on each piggy.
With syringe feeding in a pig who is eating hay but still having issues, you are aiming to keep the guts moving when they are struggling to do it on their own.
Stick in there, if you are not seeing any improvement in around a week you can always go back and speak about different options, as I know there is quite a few different options, such as probiotics etc.
After a couple of weeks of no improvement, I think the most medicines etc. my boy was on was 5, multiple times a day. I am not saying this is what will happen to you, I am just letting you know there are other options.
When making any changes I would urge you to speak to the vet first, as you would not want to accidentally make your situation worse.
Every ml counts, fingers crossed things will get easier soon, but you have to just continue to push because although he may not know it you are doing it for him.
You may want to think about making sure he is hydrated, as you are medicating likely multiple times a day, you can offer some water or syringe feed some water during medicine time. Just be aware that it may likely be thinner than his medication, so be careful how much you are giving as you do not want him to aspirate. (But if he is not noticeably drinking less, you may want to leave off doing this as it seems you already have so much on your plate, but I found it made me feel more comfortable knowing he has some water in him when he was sitting around all day long)
From start to finish my boy went between 1150g to 1034g, generally speaking a fluctuation on 30g is not really worrying, as he could have just had a big pee but I do understand the worry when a boy is ill. -
Weight Monitoring and Management
What dosages are your boy on?
You are doing so well, you have to take everyday as it comes when it comes to things like this.