Hello & welcome, I am Dr. B, a licensed veterinarian & I would like to help you with your wee one today.
Based on Reggie's signs, he sounds to be quite sore from a possible soft tissue trauma or brewing cat bite abscess on that back leg. The latter is a top suspect if he hasn't been keen to eat and feeling under the weather since bacterially infected bites can make them feel unwell. Just to note, the reluctance to go outside, fearful behaviour, and withdrawing aren't unexpected. Cats are a prey species (despite being the bane of wildlife) and when they feel that they cannot pretend all is well, they will behaviour this manner. This is because making it known outside that he is injured and vulnerable will make him a target for predation (or at least any bully cats).
Therefore, we'd want to keep him, let him rest and make sure he has access to food/water. Once his regular vet is open, a check up would be ideal since he sounds to need kitty specific pain relief (ours is cat toxic) +/- antibiotics if an abscess from a bite/scratch is found by his vet.
Kind regards,
Dr. B.
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