Please contact your state legislator about this problem. Washington State is in "mission critical breakdown" on the issue of "third party visitation rights." We used to have one of the most liberal laws in the country. That doesn't mean it was subject to abuse, but the higher courts seem to feel if might be subject to abuse (that is, too much interference with the rights of parents to raise their children) so they struck down the law. No new law has been passed to replace it, even though many other staes have successfully passed such laws. As a result, grandparents have been disenfranchised from access to the courts in truly deserving situations. It is a shame, which can apparently only be remedied by our state legislature "getting its act together."
My heart goes out to you. It is really tough to negotiate the delicate situation of your son's troubled marriage and keep up the relationship that is so vital to you and to your grandchild. The United States Supreme Court and Washington Supreme Court (you can google Troxel vistiation WA) both held that Washington's third party visitation law was unconstitutionally taking away parent's rights. Our legislature has worked hard toward passing a new, constitutional, law since then but many complex and competing interests make this law very hard to pass.
In the meantime, you do not have a way to legally force your daughter-in-law (or son) to allow you contact with your beloved grandchild. Even if you had such legal rights, it is still the very best advice to carefully navigate the relationship with both parents so your relationship with your grandchild is in harmony with the other parts of his life. Unless a child has two unfit parents (an awful situation you don't wish for), you can not force visitation.
If your son has visitation rights and spends that time with you, too, that is a good way to spend some time with your grandson. I urge you to be wise and maintain the best relationship possible with both parents for your grandson's sake. His needs must come first and yours are a very distant last (behind his and his parents' rights).