Hi PinkSunflower:
This is a really tricky area. DD1 spent much of Y6 reviewing things she new for KS2 SATs and found it very dull so I do sympathise.
With 2 years left of school ahead of you - I do think finding some means to keep your DS's keen interest in maths alive is important.
First, does the school subscribe to something like My Maths or Mathletics. If so there's no law that says you can only use this when assigned for homework. Go ahead and work your way through it. I don't know Mathletics - but with My Maths - just go to Library - have your son look at what level work he's doing and just aim to finish that Level. you have complete autonomy about what to learn next - and if a homework goes disastrously badly you know to review the lesson and try again.
Failing that - try Khan Academy - www.khanacademy.org/ - they've laid out maths curriculum by US school years. Select subjects and with your son - work out where roughly he should be - maybe pre-algebra? It's absolutely free - and there are both lessons (explaining the principle) and exercises (providing practice opportunities) + feedback to you and a kind of points system for your son.
I have to admit we've only dipped into Khan academy - I use the lessons when DD1 doesn't know how to do something as a way of clarifying the concept. We were very uncertain whether maths would be any good at her new senior school (DD1 is now Y7) - so I kept it in reserve in case once again the pace was uncomfortably slow.
If he just likes working problems - try a worksheet generator:
Math Drills: www.math-drills.com/
Worksheet Works: www.worksheetworks.com/math.html
Worksheet Works is in BETA (still in design) - both are free and will provide answer sheets.
Finally if he's getting to be Level 6 ability - he might want to try the Maths 'Numeracy level' problems (5 a day) from Corbett Maths: corbettmaths.com/ - also totally free. Numeracy Level is aimed at NC Levels 3/4/5 (GCSE G F E D grades) & there are two higher levels - Foundation (NC Level 6/7 = GCSE C or D grades) and Higher Level (NC Level 8+ - = GCSE grades B - A*). We're currently working with Numeracy Level - challinging but manageable (DD1 achieved NC L6 at KS2 SATs) - but if DD1 continues to adore working problems in future we'll be moving up to harder material and relying on Khan Academy to explain how to do things when something is new.
HTH