Soya milk for baby who is sick lots?

[2point4kids]

DS2 is being sick again. He's been sick on and off for 4 days now. He was sick for about a week recently too.
In between being sick he is happy and not unwell.
Dont think its a bug as no one else is catching it off him.

He doesnt want to eat anything when he is sick and he cant keep his milk down.

My Mum says that my younger brother had a dairy intolerance as a baby and that I should give DS2 soya milk whenever he has a bout of sickness as its not dairy so he'll keep it down and feel better...

Does that sound right? At the moment I am giving his usual formula but very watered down (so 7oz water and 3 scoops milk)