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Welcome to JustAnswer.com! My name is***** and I have been a licensed veterinarian for 10 years. I would be happy to answer your question. I'm sorry to hear Porter was hit by a car, but I'm glad to hear he seems to be healing well other than the leg swelling. This can happen due to obstruction of lymph ducts in the swollen area, especially if he is lying on the injured side. I would not worry about it for now. The most important thing right now is that he is pain and infection free. The swelling may come down as healing progresses, or your vet may recommend a course of oral steroids to bring it down.
I would just let him lie however is comfortable. It sounds like the swelling is cosmetic only so not something to be overly concerned about.