It will be classed as income, if declared, but will usually be offset against the various expenses it is intended to cover.
At 16, you could consider setting up an account for your son and paying it directly, in which case it will be his money, not hers and should not be included in her income. However, the IP may decide that your son, then, can pay something towards his keep, again, if this is declared! It is not a clear cut situation.
That said, maintenance is rarely so closely delineated. I am a single parent and I get maintenance which 'disappears' into the household expenses pot. My son, even though the money is not seperately accounted for, gets far more --- or 'costs far more' than my ex pays plus my 'share' (if you see what I mean).
Another, possible, solution would be for you to withdraw support in the form of a monthly cash contribution and, instead, purchase things as they arise, like school uniforms, leisure clothes, hobby and leisure time supplies, trips and treats, the odd household shop.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014