Ela
Although you receiving the money it is for the benefit of your son so it should not be counted as your income but I would still declare it with a covering letter explaining what it is.
Is the money paid into an account just for your son? If not then it may be worth your doing this so it is seen as being completely separate from your own finances and I would keep any receipts for purchases made on behalf of your son.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems