A great place to start is having his Doctor giving him a good physical. If he has any form of diabetes it should be easy enough for his doctor to find. Also some basic blood work can rule out a lot. After that's done only then can you really deal with ED.
For what its worth I have mild To moderate ED and cialis 5mg works great for me.
its possible first try to monitor sugar in foods better and if this still causes a problem i would def get this checked out....