You can purchase Critical Care (apple and banana flavor usually is more appetizing for the rabbits) and feed this to your rabbit, in addition to all other food. This is how Wick’s vet recommended me get him from underweight to a healthier weight.
Critical Care is usually used to help nourish rabbits recovering from a procedure (like a spay), so it’s nutritionally ok to give, even if your rabbit isn’t necessarily in recovery.
For Wick, I fed him as many syringes of Critical Care that he would want, twice a day (morning and night). I still gave him his usual veggies and pellets on top of that (at different times). You just need to be mindful that there’s a lot of time that your rabbit has to eat hay as well.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.