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You will need to look over the paw real well. Look for splinters especially around the pads. Look between the toes for sores or pebbles, redness or inflammation. If you find a pebble or splinter, you can remove it. If you find a sore, then you can treat it by soaking the paw 2-3 times a day with warm water and epson salt, disinfecting with hydrogen peroxide and applying Neosporin or triple antibiotic cream. I would then use a baby sock to keep the paw clean changing as it gets dirty.
You will also want to check the nails. Dogs whose nails have become overly long can have a lot of pain due to the nails pushing the toes back and up. Also check the nails for splits. If you find one split, you should try and cut it back. If it is badly split on most of the nail, you may want to have the Vet do it as there is a chance of some bleeding if the nail is cut too short. Take a clean dry cloth and hold it firmly on the nail tip for 5 minutes straight. You can then use a styptic pencil, corn starch or flour in a pinch to stop the bleeding though. Some people recommend using a piece of a paper bag against the nail to stop the bleeding, or rubbing a candle against the nail so the wax stops the bleeding. You'll want to keep your dog calm as walking around may start it bleeding again.
If none of these things is the cause, then you may need to have it looked at by your vet to determine if a fracture is present or not. The problem may actually be the elbow or shoulder area, so your vet can check that out as well. If there is no bleeding, buffered aspirin can be given to a dog with a dosage of up to 5-10 mg per pound every 12 hours for pain... Keep in mind that a dog's body does not metabolize aspirin in the same way as a human and thus should not be given more than a day or two without contacting your Vet. The aspirin may need to clear your dog’s system before other medications can be given, so keep that in mind if you decide to give aspirin and be sure and tell your vet when your dog is seen. Read side effects and precautions here.
If your dog is putting any weight on the leg, that is a good sign. There are a few different things that can cause this. You can read about them here
Your dog might be having problems with an elbow or with an injury to the wrist area.
If swelling develops, it might indicate carpal hyperextension which you can read about here:
Dogs can have elbow dysplasia and Lyme disease can also cause lameness as well. You can read about elbow dysplasia here:
Unfortunately, you will likely need to have him seen by your vet and have imaging done to determine the problem. I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer positively so I am compensated for my time.