I found a metal plate in my dog's front leg. I think it's hurting him. Should I take him to a vet?

Yes take him to the vet. The plate could be coming lose and if the bone is healed correctly they may be able to remove the plate. My dog has a plate in his leg also but the vet told me it wouldn't cause him any problems as he grew but if he ever showed signs of discomfort with that leg to bring him in. So yes the vet can help him.