Hi Nicola. Can you simply refuse to accept the DS for your son? Explain what you are trying to do and refuse to accept it? A perfect compromise would be to ask your ex to get games for the console instead, that way it gives Tom an added inducement to save and buy the console for himself. Or you could always take the console back to the shop and use the money to get the games yourself?
I am luckily not in the situation of being separated from the father of my children, but who knows what the future will bring, but, I do get incensed on other people's behalf when money is thrown around to prove something instead of the father being there. It isn't the presents that will be remembered, it will be the emotional blackmail that these presents represent that the child will remember once they are old enough to understand what happened.
I hope you can work something out with your ex, or find a way to circumvent his selfishness in this.
