MK, I can really empathise. It's gutting when they are so close. My own daughter is in a similar position (2/3 UMS points off the next grade boundary) in three subjects and two of these she is studying for A level too.
Some thoughts:
What does the history teacher/head of history say about it? If they are happy to let your son continue then that's a good start.
The grade achieved doesn't mean he's not good at history: he may have not answered a question particularly well, or misread something (even if he did think the papers were ok). The A level prediction will be the most important factor on the ucas form, and although these are initially predicted from gcse grades, teachers should always alter depending on evidence in testing over the course, yr 12 and mock exams etc etc.
Have you requested the teacher to view his script? This is a free service offered by most (if not all exam boards). The teacher can look at the script first, then give you a recommendation whether to re-mark or not.
My daughters English teacher told us on results day, that one language paper was v high, the other much lower - an expected result, as she started a piece of creative writing, and half way through the exam crumbled, and started again with another title

) But the teacher is going to look at this paper to see if there is any point in getting it re-marked.
So, I would ask if both papers scripts could be reviewed by a history teacher to see if both papers might have scope for an extra mark. It's a subjective based exam and unlike maths/science with definite right/wrong answers, there is more chance of different outcomes. As he is so close to the grade 7 boundary it is unlikely he will go down a grade.
If your son is passionate about History, I would still go for it. The most important thing at A level is to study subjects they enjoy, rather than ones parents/helpful friends think they should be studying! (We have a rather over bearing relative insisting my daughter take a science subject which she clearly fluked and has no interest in

)
Good luck, I hope that some extra points are found.
