I am writing from Ireland, hello!
I agree with everything Dgorgo, Theo and Morgan have said... Critical care is excellent but I only use when I know the rabbits have a tummy upset.
We have two mature 9 year old house rabbits that won't eat enough hay. We use Meadow or Timothy... Our rabbits eat too much pellets - the 70% fibre to 30% pellet ratio is perhaps the other way around in our house. However our rabbits have been able to graze fresh grass all day which does even the scales. They get fresh veggies every evening and fruit treats daily (which I know is too much, they have been spoilt over the years)
Have you tried stuffing hay into toys for your rabbit. For example a toilet roll holder with a hole cut into center where they have to work to get it out and can also enjoy a nibble of cardboard. But trying to stimulate the rabbit into eating hay by making it interesting...
As you may know rabbits teeth do not stop growing so it is critical that they are able to chew and knaw. Hay is important because while eating it they use their molars not front teeth to grind it down.
One of my rabbits has had two dental surgeries... Teeth spurs have needed to be rasped and a wobbly tooth removed. Bacteria from drool has caused an awful rash causing fur loss under and around the chin. I am finding this rash very difficult to treat because my rabbit is over grooming and this causes digestive issues.
Anyway this health issue could all be due to the fact that my bunny does not eat enough hay...
Dandelions are excellent sources of nutrition for rabbits and could help to give your rabbit a healthier appetite. I also use freshly cut lawn grass daily which they love (at the moment my garden is open and not safe for grazing)
Have you tried leafy strawberry tops, top and tail of carrots and the peel (not the carrot itself) and kale!! Your crunchie carrots are treats and you could give your rabbit a few after he has eaten some of the stuff you want him to eat... And try to reduce the treats almost entirely and use fresh apple... We use top and tail leave center out as pips are poisonous but they like the roughage too...
It will take persevering and trial and error as said. Stimulating your rabbit in general w plenty of attention and activity will probably help too!! Build cardboard castles for it and introduce games if you can...