We've been using diapers for four weeks now and they don't fit anymore…

We've been using diapers for four weeks now and they don't fit anymore (I don't know what to do). At home, the baby mostly breathes without a diaper all the time. We put on a two-hour diaper, and his bottom is red with Benpanten, Bioline cream. He's also started gaining weight and there are redness on his neck, what should we do? Potato starch