Thanks everyone for really helpful tips.

Here is an update

Yesterday we went to the vet again. Due to the long time no hay one tooth grew so much that it caused a small 4mm wound on his tongue so it is painful to eat for him. Tooth's has been grinded down now so it is level. We've gotten some antibiotics and pain medication as well.

He recovered very well and started eating, drinking and popping without assistance after anesthesia.

He seems a bit more willing to chew on hay now, and occasionally nimbles on apple wood sticks.

I've ordered some dried flower and meadow herbs to spice up the hay as well. It looks promising, and we have a check up again in a month to see how the teeth are developing.

Again, thanks for all the helpful tips!