I completely agree that six weeks is too soon, but you have no right to tell him what he can and cannot do. Of course, express your concerns, but now he knows your opinion, you just have to leave it.
I didn't meet my DSD for the first year of my relationship. Her mother causes me endless problems, and clearly feels that I have been pushed into her daughter's life far too soon. She doesn't know that DP and I were together for a year, it isn't any of her business.
The best thing you can do is look supportive; your son will pick up on your emotions and hostility, even if he doesn't know the reasons. If you son comes home upset about the new woman, then you have a valid reason to raise it with your ex. Until then, you need to smile, fill your son's life with love on your side. and let your ex get on with it.
Whilst you might have felt five months was a long enough wait, your DP met your son when he was a year old (at a guess based on the numbers you've provided). Obviously, I don't know the circumstances around your relationship ending with your ex, but that seems like a quick turn around to introduce a new partner.
Don't judge. Sit, wait, observe, then act if necessary. Who knows, this new woman might bring a bit of extra joy into your son's life. Who wouldn't want that?