Agree about tutor. I've heard of too many people recently having sons who've been turned off Maths at secondary school, despite it having been a strength at primary, because they can't access their school Maths lessons the way their teacher is teaching. Too much copying from the board/listening to oral explanations from the teacher and then being expecting to sit and work their way through abstract examples, rather than having the opportunity to explore a concept in different ways, using different concrete and pictorial representations to help make sense of the abstract. Sadly, it's not at all surprising given how many schools are struggling to recruit Maths teachers and so many many non-specialists find themselves teaching the subject. And given the pressure on school budgets, a school isn't going to put any kind of intervention in place for a child who is meeting their targets - the limited amount of support available will be focused on those who aren't making progress. So, in answer to your original question, if you want the school to do anything differently I think you are asking for the moon on a stick.
The school will have already made hard decisions about the allocation of scarce resources. They may well be aware of the problems faced by your son and others taught by his teacher. But their hands are tied - they will already be doing the best they can with the time, money and staff they have available. I'm sure the HOD won't be happy with the current situation, but if another option was out there then they would have taken it already.
What you really don't want is for him to develop anxiety about Maths, the idea that he can't do it, as that will cause even greater problems. It will become a self fulfilling prophesy - he'll get worried/panic/upset and then his brain will go into lock down and then he really won't be able to do it. So to prevent this, I can only see that you have 3 options - tutor him yourself, get a tutor for him or support him to self study (eg use websites like those listed above, or Khan academy, or hegarty maths). Only you can decide which of those options will work best for you and your son.