If he prays ‘Isha’ with Maghrib at the time of Maghrib, he can pray Witr immediately after that.
Question

                If I join Maghrib and ‘Isha’ at the time of Maghrib because of traveling, when should I pray Witr? Should I pray it straight after ‘Isha’ or wait until the time for ‘Isha’ begins?.
Praise be to Allah.
    You can pray it straight after praying ‘Isha’. 

    It says in Fataawa al-Lajnah al-Daa’imah (8/144): 
It is prescribed for 
    travellers and those who are sick to join prayers, and also for 
    non-travellers to join prayers on a rainy night. They may then pray Witr 
    immediately after praying ‘Isha’ when it is joined to Maghrib at the time of 
    Maghrib.