Is it permissible for my husband to combine the Maghrib and Isha prayers, given the demanding nature of his work that necessitates early sleep and makes it difficult for him to wake up to pray Isha at its prescribed time, which might lead to him falling asleep while driving?

The principle is to perform each prayer at its legally prescribed time, and combination (jam') is not permissible except for a valid excuse. Among the excuses that permit combination is hardship (mashaqqah). If your husband finds it difficult to perform the 'Isha prayer at its time, such as if it leads to him getting insufficient sleep or exposing him to danger during work, then it is permissible for him to combine the Maghrib and 'Isha prayers by performing them earlier (taqdim), to remove hardship and difficulty. This is based on the Almighty's saying: (Allah intends for you ease and does not intend for you hardship), and His saying: (And He has not placed upon you in religion any difficulty). It is also based on the hadith (definition) of Ibn 'Abbas, may Allah be pleased with them both, that the Prophet, peace and blessings be upon him, combined the prayers in Madinah without fear or rain, intending not to put his Ummah in difficulty. However, if he is able to wake up to perform the prayer without hardship, then combining is not permissible for him.