I think a lot depends on how your son feels about it. I think you're doing absolutely the right thing in having fun with maths with him at home. In an ideal world he'd also get challenges at school, but (as I think you realise) if he's basically content with what he's got now it would be quite easy to change it to something worse... I think in your place I'd maybe be making an appointment to talk with his maths teacher about challenging him where he is. IMNSHO, if you have a straight choice between "harder problems using the maths he knows now" and "teach him more maths, but give him only routine problems to do with it", the former wins hands down. Whether it's a realistic possibility, though, depends on how skilled the teacher is at differentiation.