I don't really have any advice here. We also experience this issue, our problem probably stemming from Gma raising all boys so our boy is nothing new to her. She dotes over her grandgirls, though, and, like the stories provided here, when he visits, the girls are there, too, getting most, if not all, of the attention.
All we can do is isolate the incidences where favoritism may occur. We no longer go there for holidays stating that we are creating our own family traditions at home. We want our son to wake up Christmas morning in his own bed with his own tree, etc. We don't leave him with the gparents where they will be also babysitting the other grandkids without first stating that "we know you are more familiar with the grandgirls but you must make a concious effort to treat the kids equal. My son's old enough to see when they are not being treated equally." I don't care who I offend, my kid's feelings take priority. If it continues, the visits stop.
This has actually helped the cousins' families visit US for a change! So, my son and his cousins still get time together without gparents- and gparents get time without the cousins around.
We also have had to make an extra effort to spend quality one-on-one time with our son when we are visiting family and to always make sure to avert the "unconcious" favoritism when it occurs.