How have you been introducing veggies? Had he had cucumbers, peppers and celery all before? How much?
Skipping usual foods can cause issues too. In terms of veggies, try to think more in terms of "greens" than actual veggies. Green leafy veggies are ideal with a small amount of cucumber OR peppers OR celery on occasion. So think about daily offerings of greens like green leaf lettuce, red leaf lettuce, romaine, basil, cilantro. Few times a week can mix in kale, spinach, parsley, or mustard greens. (That last set is high in calcium so should only be offered a couple times per week - not every day. )
Unless your rabbit has already had a particular green, it needs to be introduced slowly. For example, if lucky hasn't had spinach before, then offer him just a small piece one day. If he takes it, then offer it again the following day (nothing else new). Continue to offer the spinach each day for several days with gradually larger amounts. So long as he has no reaction, then all is good. AFter that several days, spinach can be added to his list of tolerated greens.
Each new green (or veg) needs to be introduced this same way - a little at a time and only one type at a time.
