OP, as others have said, it may just be temporary, but it might not be. Wait and see what happens after half term. If he's feeling challenged in every other subject there's no reason why he shouldn't be feeling challenged in maths too. It is possible that his teacher is not differentiating appropriately, and that may be because she's new to the role - maths teachers are hard to come by at the moment and schools are having to make do with what they can get - she may even be a non-specialist. The school website (if it's compliant with govt guidelines) should list all of the teachers' qualifications, so that should be easy to check.
If things don't improve after half term, next thing to do would be to write a friendly email to the form teacher or head of year saying how happy you are with most of the experience so far, but that you have this concern, and your child feeling unhappy about it. If they don't give you an answer that you're happy with then consider escalating it higher.
My DS had a similar experience in Y7, and when I said something the challenges did increase, along with their understanding of what he and some of his peers were capable of (based on assessments at the end of each half term). He was happier for the rest of the year.
Then we had something similar at the start of Year 8, but this time it was because he had an NQT who was capable/qualified but apparently more interested in teaching science than maths, and who wasn't able to control the class when they became restless (they were a top set and prone to pointing out her mistakes too). Again I and other parents mentioned something to the school, though it became clear they were already on to her from a performance management perspective, and she left the school before Xmas (to go and work at a private school - my heart bleeds for those who are now paying to be taught by her!). In the meantime a much more senior teacher was drafted in to teach my son's class to compensate for the dip in performance at the start of the year, so the end-of-year outcomes were very good.
Now it's started again at the beginning of Year 9, with another NQT - let's hope this one finds her feet more quickly than the last.
If your school is Good/Outstanding it should have processes in place to make sure all students are making appropriate progress in all subjects, and they will want to know if you or your son are unhappy with any aspect of the school.