My buns have always had veg since we got them, although i know some people dont feed until theyre older, 4-6 months+.
When my younger female was ill at 8 weeks old and not eating, the vet said try her on veg. She picked at bits when she was really ill, but ignored her food, then when she picked up she'd only eat veg for the first few days, and she had a LOT! Her poos werent great at first (she wasnt pooing to start anyway as she was ill and not eating), they were a little soft but only for 2 days, by day 3 they were pretty good espeically considering the amount of veg! I must emphasise this was a special case as she was ill, and i wouldnt do that normally, but my point was the right veg might not upset your bun at all.
With all my others i did it slower, but not as slow as people on here. I started with sweetheart cabbage, they all loved that! I did a piece about the size of their head for a few days, then increased it by about half the origional size. After a few days i tried a new veg, and did a similar process. I probably did it much quicker than everyone else, and people may say its not the right way, but they had not one single problem, so it worked for them! The veg i started off on in order was cabbage, carrot, watercress, rocket. Then basil and parsely. After these, i tried others but not quite as slow as i did them, i guess their guts adjusted to veg in general. Just remember not to feed regular iceberg lettuce, you can feed other darker lettuces apparenly, but i personally dont risk it, but thats choice.
Id say try a smaller piece and see how they go, no signs of soft poos and go bigger, cut back if they get soft poos, caryy on if not. I think it depends on their diet too, if they still eat a lot of hay and their food, then it shouldnt be too much of an issue, but if they cut back then problems are more likely.
I bonded my younger male with the female, and he wasnt used to veg, i gave him bits WHILST swapping him onto the new food, and no problems. I fed the veg by hand so he wouldnt eat too much, although he still came over and stole the females veg, so at more than i wanted him too, but no problems.
As some people have said, theirs may get super soft poos over the littlest things, so observe your bun. How was she/he when swapping over to the new food? If problems, then definately do the veg slowler, if not, do it slowly but maybe more than you would if you were being extra cautious.
The grass...when they are young its generally not an issue with the calcium, we get readigrass and it says feed adlib, like with the hay, but i stick to a handfull. My older bun is not interested, but the babies love it, although dont always eat it all.
When giving him the grass and veg just try and observe the hay eating, if that changes, maybe cut back on the others, if not, then keep going as you are.
