you can get grass for rabbits that grows in a little plastic tray. I always find though it goes mouldy after a while so I might be doing it wrong. you could get some small pots, soil and grass seeds to grow on rotation to make sure their is some available at all times.
tbh though, I don't mean to nitpick..but that pellet still isn't great.
I wouldn't ever advise feeding pellet morning and evening. the problem is is that some very very stubborn rabbits know that they're getting it twice a day, so they eat as little as possible and wait for their next lot of pellet. as its twice a day it allows them to sustain like this which isn't good for them. so then they eat the bare minimum as they know that their "treats" will come soon. you really have to think of candy like treats when it comes to rabbits, and not the main stable of their diet.
I would also say at 20g of pellet is quite a lot. many people's bunnies here get a tablespoon of pellet, if that. I would guess that this would weigh roughly 5g (give or take as different pellets will weigh different things).
fibrefirst in itself (while extremely healthy) would be considered a pellet or pellet replacer. many people feed these in replace of pellet themselves. I probably wouldn't give them religiously every day, more as a treat. but I'm being very nitpicky and I doubt its the cause of the problem.
hay/grass is really necessary for their dental health (as well as stomach health) so him not getting enough could cause problems down the line, but I'm probably repeating what you already know.
I would suggest reducing the pellet further and only feed a small amount once a day. they really don't need it twice a day. if he's being a little so and so, he's probably holding out and getting by with the bare minimum because he knows his pellet will come.
I'd also suggest (if you can) to feed their veg/pellet at random times. that way they don't know when their next lot of pellet is coming so they have to eat hay. for instance my two know they'll get it in the evening..but when? might be 6pm, 9pm, or 2am :lol: it keeps them on their toes and can help bunnies who are trying to hold out on eating hay.
go into PAH, they usually have deals on and cheap forage. buy multiple packs of the woodlands forage, such as bramble, dandelion root, dandelions, flowers. I'd steer clear of the dried fruit and veg however. sprinkle some of it into their hay and around their cage. it allows them to forage, which is a natural thing. and if you can get it throughout the hay, they're going to have to dig, or eat, their way to get to the "good bits".
you could also put their pellet ration in the hay - same idea as foraging. or scatter feed it so its not served to them on a silver platter and they have to work for it.
unfortunately you're going to have a battle as good hay eating starts from a young age, like a routine. the fact that he was fed a poor diet means you're "up against the tide". it can be done however.
worst/last cause scenario.... I'd suggest totally stopping veggies all together. its not totally necessary, just a nice added bonus. that way all they'll be getting is a tablespoon of pellet..and hay. at that point he'll have no choice as horrible as that sounds. sometimes its gotta be by "force".
what hays have you tried?
honestly most rabbits seem to be haysnobs, or ones that are very picky. most of the time hay in petstores are gross and nasty, brown and don't smell sweet. and who knows how long that hay has been sat there (I'd guess years). its just not "nice" hay anymore.
if you haven't, I would suggest getting hay from timothyhay.co.uk or hay-and-straw.co.uk. I know hay & straw do a sample of all three hays. they always smell sweet and depending on the type or cut, aren't a dark dead looking brown. the ings hay has flowers in there sometimes. I'm sure timothy hay would do samples too. as they're farm grown hays and from last years cut, they're "fresher". timothy hay also has more flexibility over what you can get, soft cut, stalky cut - some rabbits like a particular cut.
or if there are any other pet stores around you, perhaps smaller ones, I'd stop in there. sometimes the smaller stores source their hay directly from farmers, so that can be nice and fresh hay. ask them, some just have the commercial stuff, but some will source hay from local farmers. also helps a local business
it also cuts out the issue of transporting a huge bale from farm to your house if you don't have a car (and the mess of it!) as its (usually) bagged.
equine stores can be good too, they often have fresher hay in bulk. some will allow you to take samples home first to see what bun bun likes.
or my last idea is see if there are nearby farms and if you have a car, pick up a bale from there. usually fresher and better quality too. not always but it might be worth a shot.