Mine get this too sometimes, it's not really anything to worry about unless it cracks so badly it bleeds or is causing him discomfort or irritation. You can rub olive oil or vaseline on there to keep in moisture and I think you can also get stuff called paw wax which protects their feet although I've never used it. If you're doing a lot of road walking in the winter months it could be worth rinsing his feet in clean warm water after walks to remove road grit/salt etc. which can irritate their feet, as can run off in puddles from chemicals like de-icers which are used a lot at this time of year.