First and foremost grand parenting is not a competition unless you want to make it one.
I do have some empathy with your situation. When we first became a GP dh was the worlds best authority on child rearing. (Not)
Until I reminded him
1) She is their child and it's their family.
2) I did most of the parenting as he was at work so where sis he get all this knowledge and childrearing experience from?
3) His childhood was so awful (according to him) why on this good earth would I want to repeat that with our girls and certainly not with the DGD.
Therefore, I'd start by asking you dh how pro-active was HE in making sure HIS mother and father got to see their GC?
Then ask yourself how pro-active were you in making sure you MIL and FIL got to see their GC ?
Are you, as GP, really any better or worse off than your in-laws were?
Did your parents get to see more of your children than his parents?
These questions are there to help you and your dh get some real perspective on the situation.
Because it's a big thing to ask someone to be more thoughtful with us than we were with our parents/ in-laws.
If you and your dh were more thoughtful around GP's visiting your children, their GC, than may be that set up certain expectations. If so that was you then, this is your son's family now.
Ask you dh how much support, encouragement, and general "you're doing a good job" has your DIL had from him ? Or from your son?
Most men (again not all) left the child care to the women and how many of our husbands ever said to us 'WOW you are doing an amazing job of raising our children, being the housekeeper, worker, mum's taxi .... ?
Only since retirement and watching our DD and SIL doing what's involved in keeping a family working has my oh realised what it took to keep it all going. It was my Mum, not my Dad or PIL that said I was doing OK, that I'd done somethings well, that I might try doing somethings differently. Mum's can do those thing, say those things that others often can't. It's just one of those life quirks.
As GP's we often (not always) have more time on our hands than we did when we were parents.
We need to remember what it was like when we were trying to juggle all the things our AC are juggling, parenting, work, our relationship, friendships, disrupted sleep etc, etc.
I know with my parents and in-laws we had to make "Arrangements". I never thought of them as "Appointments".
I had to make arrangements if they were coming to see us or if we were going to see them so this doesn't seem so strange to me.
I suppose if you/they live very close and talk often about what other arrangements are involved in each others everyday life then may be arranging a time isn't as important but not many of us live like that.
Bottom line as long as your son and DIL aren't being out and out rude or withholding then your dh needs to realise that you and he are only one piece of their life jigsaw.
They are their own people, with their own needs and priorities, living their own life in the best way they can and NO ONE was put on this earth just to out and out please another person.
We are not here just to knowingly hurt other people but our lives really are not all about living in such a way that our parents, our in-laws, our siblings or anyone else's is not put out.
I think you have a better perspective on the situation but it really does came down to it's their child and your DIL will be drawn to who she knows more, her Mum, where she feels more comfortable and where she doesn't "feel" any implied criticism or disapproval.
I wish you well and enjoy your DGS