I came across this a few times when working with families, and also in my extended family. I think that there are 2 thigs to consider:
I think you need an open conversation with your son's family. It sounds like you all get on well, so you can explain.
To help you in that conversation, you need to know, as far as you can, what relationship the children's dad and his family have with your step-grandchildren.
You would behave very differently in these 2 extremes:
Your step-GCs have no contact at all with their birth father's family and are seen by your son as his children.
Or: Your step-GCs have a close relationship with their birth father and his parents, and are bought nice presents by them.
I have known this compromise, but of course it depends on your own resources and gift-buying habits:
A mid-cost gift (or money) is given to all of the children at Xmas & birthdays. The 'top-up' for the birth grandchild is put into a savings account.
Whatever you decide, if it done openly and with love, then everyone usually understands. The step-GCs are of an age to understand, and will probably appreciate any recognition.
As I type this, I realise that I don't know if any of your existing grandchildren are your son's. I don't think it alters the principle, but may mean it's a little more awkward in practice.
I would also say that your step-GCs are fortunate that you are thinking of them - I have experience of the situation described by aonk!