A. You can't. No good will come of your broaching the delicate topic. That's a job for your husband, who will sound much less accusatory than a protective, sensitive daughter-in-law.
But before you send your husband off for that potentially awkward sit-down, recognize that while it would be great if your in-laws were lovey-dovey with their stepgrand, it's not all that weird that they're not. After all, loving and accepting someone who doesn't have blood ties can be tough for some people, even if said someone is a child. For sure, my dad is nice to Nick's son, Mazi, and talks to him about school and football and topics to which both of them can relate, but when it comes to Mari and Lila? They get the special grandchildren treatment -- rides on the knee, walks to the park, trips to the ice cream store, little gifts just for being. Nick and Mazi understand it and accept that it is what it is.
Bear in mind that your oldest has other grandparents -- your folks and his father's -- so your husband's parents might be worried that it's not quite their place to dote.
Now, if your in-laws are downright mean to your firstborn, then by all means a talk should be had. But if they're just less effusive, and your son has other grandparents to spoil him, leave it alone.