C.M.
Speaking from experience, we tend to be a distance from both sides as well. Having a new one tends to excuse you from having to travel. Trying to tote a small child and packng up all the things you need and don't, just to visit can be hard on parents and kids. It is not your job to make your inlaws bond with your son.
You might want to have your husband voice concerns about their lack of involvement. Some times they tend not to notice, or are worried about stepping in where they shouldn't. It is always good to see where they are coming from first, as they may handle things differently then your family.
However, if it is a matter of convience for them. This is their grandchild. They should be around. Just be careful of always going out of your way, after awhile it gets to be expected. And becomes a hard thing to turn around.
I wish you good luck, and just remember no matter how much or little they are around, you have all the love your son needs to be happy.
~ C. ~