It will NOT change his stats when he levels up. Yes, it is a good way to train a low level pokemon. Fight the highest level pokemon you can find with your 50+ and Growlithe will be levelling up there fast! However, once he gets up there in his level, you will want to start fighting with him to quicken it up. But save the Exp. Share, because you will probably use it again! And remember, it will NOT affect his stats in a bad way when he levels up, he will level up and evolve like normal.