This is one of those situations where your instinct is to be able to offer practical support - like making tea in an air raid, but the reality is, there is very little we can do when this sort of scenario occurs.
The British are seriously bad at death and grief. We like to do the solomen bit, have the funeral, and three weeks later, get back to normal with a sigh of relief.
Of course, it is not like that. Your partner's grief will be long and difficult - some days he will not mention it, other days it will possess him completely, and time may be the 'great healer', but time is what it takes for the earth to spin into the sun - no-one knows how long grief will take.
So - you come into your own when everyone else is being really 'British', and not mentioning your partner's dada, and he will need someone to laugh, and cry with about his thoughts and feelings. That will be you.
Make it clear you are there to listen if he wants, but without pressure. beware of making yourself feel better by making yourself 'needed' - it wonlt help either of you, or your relationship.
Be intuitive, which women are so much better at than men. Read the signs, and he will get to a level where he can assimilate what has happened, and move on.
People don't 'get over' the death of a loved one, the weave the loss into the fabric of their lives, and carry on living, som days it hurst more than others, and that can go on for the rest of his life.
You are obviously a very caring person - your willingness to travel this road with your partner and his family will be seen and appreciated, even if not acknowledged, and everyone will get past this terrible first stage ... eventually.
Come back if you need to - we are all here.