Oh MAS, I am sorry for you, DH and DS
I have been in exactly the same postion as you. 3 years ago DFIL passed away very suddenly and unexpectedly. He was fantastic and DS1 had such a brilliant realtionship with him. DS1 was 11 too.
I found it very hard, as you do, as although he wasn't my dad/grandad, I was very fond of him. But I needed to be there for DH and DS first and foremost (Ds's 2 and 3 were only 2.6 and 6 months so didnt understand)
I feel I was so occupied with making sure DH and DS were alright that I didn't give myself time to grieve and it hit me hard a few months later.
I think all you can do is be there for them, hold them, cry with them. Talk about your FIL. It helped to talk to DS about the funny things he did with his grandad. He said he loved to remember him for the times he had with him...cycling through the drivethrough in McD's....being wheeled in a wheelbarrow up in laws lane with a torch to meet DH because the lane had flooded. He has many memeories. DS chose not to go to the funeral because of this, he decided he didn't want to have his last memory of grandad being his funeral.
When I have seen you speak of your DS, I feel he is very much like DS1 and will respond well to chats about grandad.
It will take time, but you will get to a point where you can have fond and happy memories about your FIL. 3 years on for us, we still talk about him and laugh about the stuff he did.
Take care x