Hi fluffy my DS1 started to stammer overnight when he was 2. I was very alarmed and it was so unexpected as we don't have any other people with stammer in the family and it is often hereditary.
As a parent i have found that information is power - the Michael Palin Centre for Stammering Children has a fab website and so does the British Stammering Association - they will send you an info pack and have a helpline.
For some children it is "just a phase" and the number of children who go on to live with stammer in adulthood is a small percentage. Early speech therapy is thought to help - but this is one of the areas that has been massively cut in recent years. There is also loads you can do as a parent from slowing g your own speech to encouraging turn taking in chats and having 121 time. (All in the leaflets/website)
My DS1 is 9 now and yes, he stammers. It comes and goes, he can be perfectly fluent and then get tired and its back. However we taught him from an early age that WHAT he has to say is so important that a stammer doesn't matter. We have used the examples of Winston Churchill, Ed Balls, King George etc. We have given info packs to his teachers, cub scouts, rugby coaches and helped him practice responses to (often totally innocent ) enquiries from other children. We have educated grandparents on what is/isn't helpful.
I'd be lying if I said I didn't wish he didn't stammer. Coming to terms with it myself and the fear of the future (will he be able to cope with a job interview? Is it my fault?) has been hard. However its all about attitude - My son stood in from of the whole school and read a passage at Christmas and every single teacher had a tear in their eye. He read it perfectly but that wasn't the point - he read it knowing he might stammer, caring very much if he did, but being brave enough anyway.
Please feel free to PM me if you want. The organisations above are really good in my experience and should be able to give you lots of info. Lots of love to you and your DS