Incisor malocclusion in a young rabbit is most often due to genetics – ie it is not caused by eating to little hay. That makes sense, because rabbits only use their incisors to pull hay into their mouth, not to actually chew the hay. They grind the hay with their molars.
http://www.mypetsdentist.com/rabbit-incisor-malocclusion.pml
A vet can clip or burr the incisors to the right shape. It’s not sth you should do yourself, because there’s a big risk of causing cracks or in some other way making the problem worse if you’re not experienced/trained.
I don’t know if your bun’s incisors are “bad” enough to warrant a trim. Anyway, this is NOT sth you caused.
As for hay, it can be tricky to get them to eat enough. They need it for their tummies and for their molars. My buns like oat, other buns prefer orchard or botanical. All types of grassy hay are equal in fibre, it’s just alfalfa hay you should avoid (too rich for a bun that’s not a baby). You often have to try many types of hay before you find one that your bun likes.
The incisor-problem might be the reason why he’s not keen on eating sticks or twigs. Or you just haven’t found sth he likes. My buns love apple twigs and willow. If you get fresh twigs, make sure they’re not treated with pesticides.