Well, it sounds as if something in the relatively recent past may have bothered not just your DIL but both of them--or possibly not; perhaps your son hasn't changed his personality entirely but is juggling being a parent, being a worker, and being a spouse, and being a son isn't in the forefront of his priorities (which is how it should be, in my opinion). Be wary of making the assumption that a daughter-in-law is controlling her adult, fully functioning husband. It may be the case, but guessing that it's the DIL's fault can blind a person to other reasons for a drop-off of contact.
You mentioned that you invited yourselves over twice; could that have bothered them? How long were your visits? Was it possible for them to tell you it wasn't a good idea at the moment, for whatever reason, or was that something you might have brushed off or been angry about? Very occasionally, visiting grandparents expect too much of their over-stretched offspring.
Often, after the birth of a baby, parents just don't travel much for a while, in part because physicians recommend that babies spent limited time in car seats. Do they have a lot of time off, or would the 6 hours in a car happen over the course of a Saturday and Sunday? That may be more than a pediatrician would recommend. It wasn't when I was a new mother, but it is now.
You could ask your son if there is anything you've done to damage your relationship, but only if you're willing to listen to what he says and not blow up on him. It may be, however, that having a baby has made their lives so much busier and more complicated that they're stretched thin. It may have nothing whatsoever to do with. For all you know, your daughter-in-law may have post-partum depression; probably not, but if she does, that might factor into things. On the other hand, my own husband felt like he'd been hit with a whammy when our son was born; he just hadn't expected what a change it would make in our lives.
In the "Relationships" part of this website there is advice on being a grandparent from a distance and on forming a strong relationship with a daughter-in-law; perhaps you might find something helpful there. Sending them some cheerful and undemanding cards or postcards might be helpful.