How is he acting otherwise? Is he grinding his teeth with pain? Is he showing interest in food?
Given this happened after he jumped about vigorously, it sounds like he has injured his back. It would be worth getting him seen to find out exactly what is causing the loss of use of his legs. It may be just swelling, or something more like fracture. This is not necessarily a death sentence, so be sure to see a rabbit savvy vet. Even him going on some anti-inflammatories may help him gain some function of his legs again.
This is an excerpt from a long article about hind end weakness in rabbits. It’s a condition that can have various causes but this is just the section about trauma.
Trauma
Cause: Damage to a rabbit’s back by any kind of trauma can lead to partial or complete paralysis of the hind limbs. The most common cause of back trauma occurs during restraint of a rabbit when he kicks out suddenly or twists. The more forceful the restraint is, the more likely it is that a spinal injury will result. The force of the kicking or twisting can fracture vertebrae in the back, which are then unstable and result in severe bruising or severing of the spinal cord. Rabbits can also sustain this kind of trauma (although rarely) when running or playing. Rabbits who are confined to cages and not allowed daily periods of exercise are at an increased risk for trauma due to decreased bone density.
Signs: Complete or partial paralysis is immediately evident after the injury. There may be loss of bladder and bowel control.
Diagnosis: The damaged vertebrae can usually be seen on an x-ray. Occasionally the vertebrae will “snap” out of place during the injury, cause damage to the spinal cord and then go back into place by the time the x-ray is taken. These cases can be difficult to diagnose unless high-detail x-ray film is used or a myelogram is performed.
Prevention: Preventing damage to the spine from restraint is two fold. One is to make sure your rabbit has an hour or more of exercise daily outside of the cage where he can move quickly, or jump up and down off of low surfaces. This will help to maintain bone density. Secondly, learn how to restrain your rabbit properly with a minimum of force. Desensitize your rabbit to being held and handled by gradually teaching him that touch is good; train with treats and use short sessions. Work on the floor for anything that might be particularly scary to prevent injury from jumping off a high surface or out of your arms. If the rabbit is struggling a lot, then release him, let him get calm, and try again with less restraint.
Treatment: If the spinal cord is completely severed or seriously bruised, there is no treatment that will return normal neurological function. Euthanasia should be considered for these patients because their quality of life will be poor. Rabbits who have only mild to moderate damage to the spinal cord or who still have sensations in the toes and maintain bladder or bowel control have a chance of healing. These rabbits should be confined to a cage for a period of 6 to 8 weeks to facilitate healing the fractured bones. Corticosteroids may be used for the first few days after the injury. Many of these rabbits will regain at least partial if not total neurological function and live a fairly normal life.
source: https://www.veterinarypartner.com/Content.plx?P=A&A=490&S=5
I just want to mention: some vets will not want to use corticosteriods on a rabbit. They can be powerful and fast acting but need to be used with caution in rabbits. It may only be one or two shots they’ll try early on in treatment.
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