What dose is he on and when is he taking it?
Sometimes breaking up the dose helps, and they also have an extended release which has been more gentle on the tummy for some people.
It was awful for me... and even months in when I got over the initial GI upset, I started getting stomach pains and had to stop taking it.
Also, if he does continue on the metformin for a long time, make sure he either takes a B12 supplement or has his levels checked - it's a common side effect. No one ever told me it could happen, I found out years later I was B12 deficient and had to get injections for a while.