Some interesting view points on this thread, but predictable I guess for those who've been around a while.
Very best of luck to your DS @Sanddancing with his decision making. There's some similarly academic YP on these language combination threads who have progressed with Arabic in their programmes and great information from posters already received on their experiences.
I don't know anything about IR, so I hope you find the right combination that works for him. We considered a number of factors, that included not only access to the year abroad but what exactly was on offer. Having no Arab connectivity ourselves, DD wanted to ensure that both her subjects were able to be studied for the year, her A level Spanish already being at high fluency level.Those that took a French/Arabic combo did not have access to Arabic during their split year in France, and as a consequence, many have now dropped Arabic in their returning 4th year, whereas mine had her 2nd semester delivered totally in Spanish, including her Arabic lessons (not so easy for someone who is British!) and has continued both in Y4. But, there is early term confusion with programme delivery, given that some went to Morocco, others to Egypt. Previously, Beirut would have also been on the access list, so the teaching back at DD's university for the returners is toggling between all, not ideal, but it is what it is and will hopefully improve as the final year progresses.
For some like mentioned on here, dialect and version of Arabic will be important, and having a university that thinks and communicates this well to their prospective students is important. My personal opinion is that Exeter has the most joined up approach to their programme options, with Edinburgh delivering well too. But this is anecdotal. DD is learning Egyptian and MSA, and says it's getting easier to converse across the others now.
Work backwards on the level of Arabic your DS wants to achieve. Is it to intellectually stretch him during UG study due to having an innate ability to pick up languages (I get this, DD did similar GCSE Mandarin, French and Spanish), does he want the option to use it professionally, develop deeper regional understanding culturally/politically/economically etc - all areas to probe each university option more on modules available to meet his individual needs.
My final personal feedback as he is clearly very bright and able, is that he should find similar students around him, given that it's not the easiest option in the first place. It's been a wonderful learning experience for mine, such a fascinating programme of study with a great cohort of likeminded students, home and abroad with similar outlooks to life.