I know they should tell him but how do I force them when they're, quite obviously, scared??
You can't 'force' them to tell H how they truly feel. What you can do is to continue to be the safe haven to the boys so they feel safe with you emotionally and can confide their worries/concerns to you. One other thing you might want to consider is setting up family therapy for everyone involved so, in this way, the therapist can facilitate the opening up process in addition to shielding the boys and you from H's wrath as a neutral party.
What do you think?
If H isn't open to the idea of family therapy, then you might want to consider putting that in writing as a part of settlement/Court order.
