Locomotor -
first off you shouldn't blame the teacher or supply teacher for the structure of the class - this will have been a decision made by Senior Management.
second - you can ask the supply teacher how they distinguish easier tasks from more challenging tasks - for instance DD2's (Y5) school labels work as hard/ tricky/ will melt your brain! - all three types of work on placed on all tables and children are encouraged to have a go. A huge fuss is made of you if you tackle a 'will melt your brain' problem and get it right.
If you can understand that the more challenging problems are on green paper (for example) or have a special name - you can suggest to your DC or just regularly ask if he tried any of the really tricky problems.
At first I was a little concerned by mixed-ability classes but in all sincerity if done well - mixed ability teaching (so everyone taught the overall concept) and then mixed ability work - so pupils encouraged to try the next sheet with harder problems or indeed seated in such a way that the teacher has organised them into ability groups - it can rub along nicely.
The good side of this is that no child is made to feel they're 'bottom table' and 'no good at maths' and there's an open invitation to try more challenging work (i.e. no child is excluded from the full range of work that is available on that topic). DD1 languished in bottom tables in maths for close to 4 school years - she was constantly saying her table wasn't allowed to work with actual numbers in Year R and Year 1 and in Year 2/3 her table wasn't allowed to try multiplication or simple whole number division. So trust me - sets aren't the be all and end all of teaching methods either.
It also avoids that age old problem of 'pigeon holing' pupils and thinking you're bottom table and don't know your times tables so therefore won't get perimeter/ area problems - which is an assumption and isn't always true.
Now - if you feel the pace is too slow for your DC - which is an entirely separate issue - there are solutions.
Does your school belong to an on-line maths programme - like My Maths or mathletics? If so - then your DC can simply do more at home - extending his knowledge by learning new things or reinforcing ability/ improving speed of recall by practising things through playing maths games.
alternatively - you can investigate the huge range of on-line maths tutorials (mathletics/ maths whizz/ komodo maths/ mathsfactor/ Khan Academy). There's a lot out there - so if you aren't happy with what is happening at school this year (and a teacher off ill long-term and supply teachers not particularly moving things forward - which is a failure of school senior management not them by the way) - you can simply do more at home.
HTH