I'm sorry to hear that Leo has injured himself.
You can safely give him plain aspirin at a dose of 5-10mg per pound every 12 hours. Just make sure you only give plain aspirin. Stay away from the newer over the counter pain relievers like acetomenophen, ibuprofen, and naproxin. It does sound like this is something that will pass if he is using. I would suspect a soft tissue injury that will heal with time.