Severe stomach pain that goes through to the back is a symptom of pancreatitis. I was hospitalised for ten days with these symptoms and was diagnosed with idiopathic pancreatitis.
Standard Metformin is known to have side effects of upset stomach, diarrhea, flatulence etc... If this is what your dad is experiencing then maybe the increase in Metformin is causing it. He could try asking for Metformin Slow Release which is better on the stomach.
Diabetes itself is not known to cause severe stomach/back pain.
Suggest your dad goes back to the GP about the pain.