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

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, and blessings and peace be upon the Messenger of 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.