Not even a Psychology degree will have A level Psychology as a requirement. But equally, in several years spent looking, out of idle curiosity, at the the entry requirements for various degrees (3 DC, studied Biochemistry, studying IR with a MFL, previously interested in studying Architecture but now intending to study History of Art, resoectively) I have yet to see requirements for anything which state that offering Psychology will mean automatic rejection even if the applicant otherwise fully meets the requirements and would have been accepted, if only Psychology had not been taken alongside them. Psychology really isn't General Studies or Critical Thinking (the two subjects most commonly not a accepted as one of the number of A levels required for a degree course and even they are not an automatic rejection if requirements are otherwise met, just discounted).
There are courses requiring two sciences, which do not accept Psychology as one of them, but even then, the third subject is unlikely to be specified. And e.g. your DS won't get any offers for Medicine, except possibly from Newcastle (which has no specific subject requirements), but because he isn't offering Chemistry or Biology, not because he is offering Psychology - but presumably this isn't his goal, anyway.
If he is only going to be doing FM as a third, not 4th, A level, though, he should check that this will be accepted for any degree course in which he thinks he may be interested.