Both DD are doing a humanities subject + Arabic from scratch.
From memory, there are about 10 unis in the UK which offer Arabic. A number have been mentioned here (Edinburgh, Durham, Exeter, Leeds, Oxbridge). Also, SOAS, Warwick and Manchester.
One DD is at Edinburgh and loving it - is now on her 3rd year abroad in an Arabic speaking country. The other has just started at Durham and is doing her combo (which was strangely hard to achieve) under the Liberal Arts rubric.
I’m not sure there’s room in a degree for your son to do IR + French + Arabic de novo if he wants to have the full Arabic language experience.
I looked into this a lot for both daughters, especially younger DD. Arabic seems to take up more of the curriculum than other languages.
So, for example, my DD at Edinburgh only had room in her timetable for history + Arabic (she had been hoping to take some French language optional credits). She was unable to take any optional credits whereas the other history + MFL courses did have room for optional credits.
Similarly my younger DD is studying philosophy + Arabic – only 4 universities where she could’ve done this particular combination in the UK (Durham, Exeter, SOAS and St Andrew) whereas it is possible to do philosophy + other MFLs at a large number of universities.
I can’t remember if you’ve said that he’s looking at St Andrews but that may be a place where he could do all 3 subjects. It’s philosophy + Arabic degree is a 5-year degree with the year abroad. (That said, it’s very hard to get into - DD was predicted A star, A star, A and didn’t get an offer but her A prediction was in philosophy and i think that may have made a difference. She didn’t really mind as she wanted a 4-year degree, not 5).
Good luck - exciting times!