How to help dog with sore pad?

I walk my dog daily and in the summer I try to go early in the morning or in the evening. I was walking him mid-morning the other day, later than usual, and the next day he had a limp in his front paw. A friend with veterinary experience looked at it and said his pad looked damaged and maybe I had walked him on too hot concrete. I let him rest for 3 days and he continually seemed better, then yesterday I walked him and he seemed fine. He also ran around at the park (on grass) with no problems. Today though I went to walk him and the limp is back. I'm guessing I just tried to start back too quick and should've let him keep resting. Do you think that's the case? I've checked in his paw and what not and can't find anything stuck in it or any cuts or anything. help please!