Doh, that will be seven months then...sorry, at home on codeine and I'm going to blame them for scattiness!!
Apparently milk isn't the best idea as its a bit too rich. Gastroentiritis upsets the balance of bacteria in the gut so it can take them a while to get over it. Dairy doesn't help this process.
At seven months I assume you have / are weaning - if you think s/he is hungry, try baby rice made up with water instead of milk, and mix it very runny so its easy to digest and more importantly, easy to bring back up again!
I started on this then had to progress to what the HV called a blonde diet, completely dairy free, he just ate chicken, potato, pasta, rice, porridge made with water, fruit and veg puree. I watered the above down until I was sure I was getting something out.
If its very early stages and the main problem is dehydration then you must force water on your LO. DS was refusing to drink a week ago and HV said mum knows best, 5mls every 5 mins. But this was after nearly 18 hours with no wet nappy.
Hope this is helpful, however unlike some mumsnetters, I'm just a mum, with no medical quals whatsoever - ring your HV or GP for more help.