Ideas on young grand children meeting new "step"gram pa

Just introduce him without making a fuss. Kids are usually fine with change and new people. It'd adults who seem to make a problem/drama out of it.

Yes, no fuss, just include him on an outing or something and introduce him as “John” or whatever his name is.
The important thing is not to force things, no “Aren’t you going to give John a kiss too?!”, or insisting he gets a grandpa name.
My grandma’s boyfriend was “Uncle Firstname” same situation for my childhood best friend. That was nearly 50 years ago and I think just the first name would probably be more normal now.
Also your grandchildren would probably prefer to still get some time with just you and not have you always come as a package.
Children are very adaptable, they probably won’t bat an eyelid.

Sorry, to be clear, my grandma wasn’t married to my “Uncle Steve” but my best friend’s granny was married to “Uncle Ted”. I think insisting that step parents or grandparents are called “mummy/daddy/nanny” - can be a source of tension. If it happens naturally and the parents don’t mind then fine, but otherwise steer clear.

This thread is a duplicate of one the OP posted a weeks or so ago

I recall that most Grans suggested you just call him by his name as your Grandson he is still in touch with your ex husband.