Twinklyfaerieglade there is always so much more on here when people post than the words that they use, some have been very quick to judge you.
I am sorry for your loss.
My only suggestion is to ask your son if there is anything you could do to bring him any comfort at this time.
He may wish to send back a silk rose to be put on the coffin (not sure what the laws are about taking flowers back from Oz, I was stopped from taking a mango from one state to another somewhere!).
Maybe toasting your FIL with a drink and mentioning your son at the funeral wake, I don't know. I don't want to trivialize it.
My father died suddenly 12 years ago and we toasted him a fair bit in the early days.
My mother died last year and it was expected and, to some extent, welcomed as she was very ill, so there seems to have been less need to 'remember her' in a way, since her passing was much more expected.
I do feel death is very, very personal and how one marks it really is down to the individual.
I really hope this visit will change things between you and your son and maybe in some small way your sympathy and visit at this time will build a bridge.