If he really can't face telling his dad face to face then maybe a letter or a phone call while you are there to mediate with your dh.

Given the issues that Gay people still face every day with discrimination and inequality I can understand why a parent would be concerned. Nobody wants too have thier child have to deal with additional potential hardships in life. So it may be that this is where your dh is coming from. Obviously a supportive family can help negate some of this and helping your dh understand this is so important.