DS (10) is working just below the national average in maths. Should I be concerned? he's national average everywhere else. I'm also crap at maths but I'd like him to at least get a 4b on his year 6 sats as I know for a fact the secondary school stream the first year depending on sats results.
Y5 (I'm assuming he is aged 10) is usually about 3A or 4C. If he's a 3B I wouldn't be too concerned as children may be awarded a 'level' but have 'elements' of the next sub-level but needs to show consistency in his work.
I taught Y5 three years ago and if I child was working even at 3C or level 2, I would be considering extra support. Talk to his teacher about his difficulties and what support the school are putting in place and what you can do at home to help, without giving him a complex or turning him off learning altogether!