My husband commutes daily between Johor and Singapore. Can he combine (jamak) his prayers because of that journey?

Usually not, because that short daily journey does not reach the travelling distance that permits combining (jamak) and shortening (qasar) prayers according to the Syafi'i school. Crossing a national border is not the measure; what counts is the distance travelled. So Zuhur, Asar and Maghrib must each be performed in its own time. Plan places to pray through the day, such as the surau at work or a mosque near the route home. If your husband is often stuck in traffic for a long time until the prayer time is almost over, ask the state mufti's office about that particular situation, because there are details that depend on circumstances. See prayer guide.