Magnesium-Diarrhea

Hello My son had his LVAD Installed 12/15/16 he's been having problems absorbing magnesium in his body so they increased the amount of mag/oxide to 1200 mg 4x per day along with slow mag in hopes that his body would hold more magnesium. This has caused him to have diarrhea for several days. When we told his LVAD team about this they blamed the magnesium and pretty much left it at that. This can't be normal and thank god he drinks lots of water. Has anyone else experienced this? What did you do to stop the diarrhea? Any thoughts please help. Thanks