Headoverheels:
This is really difficult - in general parents of able pupils asking for more tend not to be particularly well received IME.
My advice would be to try and create an outlet for that interest in maths at home - thereby encouraging your child and providing practice.
Ideas:
NRICH Maths - brilliant maths site - where children are encouraged to think through puzzles and solve them (answers are provided) but you can also tackle a puzzle without an answer and submit your answer on-line. Website: nrich.maths.org/frontpage - start with upper primary - if that's too easy try lower secondary.
If your school belongs to My Maths - encourage your son to play the games going for high scores - this will post his name & school. You can capture the screen and print it out to put up his 'records' on the fridge!
Woodlands Junior School Maths Zones - has tons of games - all free: resources.woodlands-junior.kent.sch.uk/maths/
Math champs: www.mathschamps.co.uk/#home
Now there are a number of on-line maths tutorials which many here on MN have praised: Komodo Maths/ Mathsletics/ Maths Whizz/ Mathsfactor/ Khan Academy - most are by subscription but Khan academy is entirely free. They all have very different feels and it may be an idea to explore them and then show the one or two that seem relevant to your son.
If your son fancies a little bit of daily challenge - he could try Corbett Maths 5 a day: corbettmaths.com/5-a-day/ - this is for secondary students in preparation for GCSE Maths - but the 'numeracy' level would be appropriate and sometimes challenging for an able Y5 working at Y6 level. This is entirely free and answers (showing how it was worked out) are provided.
There are a number of free worksheet websites:
Mathdrills: www.math-drills.com/ - answer sheets can be printed too - you just have to choose the subject and then explore the various sheets which get progressively harder (shown as letters of alphabet) - it takes a bit of exploring but can help.
worksheet works: www.worksheetworks.com/math.html - this is a US website still under design (BETA) - but you can control how many problems & difficulty of problems and answer sheets are also provided.
Finally - I'd advice you to just look over the various things covered by the new curriculum for maths: www.gov.uk/government/uploads/system/uploads/attachment_data/file/335158/PRIMARY_national_curriculum_-_Mathematics_220714.pdf - just scroll down to Year 5 and Year 6. It is worthwhile just double checking that the school is covering the full range of the curriculum. If there are gaps - try searching for resources - just type resources binary numbers (for example) and see what comes up. It can take a bit of hunting. You can also use the search engine within Khan academy - which often will have videos explaining the concept.
If you feel your son is ready for algebra - DD1 really enjoyed mathsfactor algebra school - I don't think you can just join it on its own any more - but it may be worth considering if he's pretty well mastered decimals/ percentages/ proportions/ ratios/ long division & multiplication/ area of 2D shapes & volume of 3D shapes. It's now here: www.themathsfactor.com/ - under games & more.
HTH