Also put his name down at affiliated Chepstow House, it's a co-ed pre-prep where children are guaranteed transfer to Wetherby and it tends to be less oversubscribed than Wetherby. Also, Hawkesdown House is an excellent boys' pre-prep in Kensington that should have a definite place still.
Yes, there's a chance - contact the school and they'll be able to tell you.
And yes, there is always movement on waiting lists, as families leave London and so the drop out rate is pretty high. But people do forget to tell schools they are no longer interested, so the shuffling time can be (nail-bitingly) close to start dates.
You will need to look at other schools too. As your DS is so young, it's very difficult to predict the kind of environment you will actually want when he is 3ish, irrespective of how good it may look to you right now.
My DH and I have recently moved to Notting Hill from Paris, my DS is 4 months old and we've just begun looking into schools in the area, Wetherby is our first choice but looking at the website, boys should be registered a few days/weeks after birth to be placed on the definite list, my DH is 4 months old so, is there still a chance he could be placed on the Definite list? and if he isn't how good are his chances if he's placed on the waiting list?