Hi there - thank you I have just seen your post.
It's very often the case that with a molar spur the teeth are not grinding against one another efficiently and so trying to get a rabbit to eat hay is like trying to cut hay with blunt scissors - it just doesn't work. Filing down the spur will make a difference, and time will tell whether he will get around to eating lots of hay.
Readigrass is a tasty alternative to hay, it's good for the teeth, just as much as hay, and tastier to most rabbits
Metacam - make sure the vet has given you the correct dosage carefully. There is Cat Metacam and Dog Metacam and they are dosed differently from one another.
Here's some info on grass and hay - fresh grass is also very good, if he will eat that instead of hay?
http://www.therabbithouse.com/diet/grass-hay.asp
This link also shows you how hay and grass influence tooth wear:
http://www.therabbithouse.com/blog/2011/10/07/grass-hay-tooth-wear/
It's a side to side grinding action for hay and grass, which wears the surfaces of the teeth. Pellets are often easier to eat as they are an up and down munching motion but don't wear down the constantly growing teeth surfaces
When did the vey suggest giving him a dental? I expect he wants to get him eating better first? xx