M.F.
K., there are no bad guys here.
Sure, it would've been nice if your husband was reading from the perfect PC movie script that would intruct him to hold you and tell you how much he sympathizes and then maybe give you a backrub (later in the film you would win the lottery or something, problem solved!)...but back to reality....
To be fair to him, he knows that your child is being cared for by the woman who raised him, so it's likely he is comforted by that fact even more that you are. Further, few men have ANY expectation of being home to raise their children, so he probably CAN'T realisitically sympathize on the level you wish.
But that doesn't diminish your feelings AT ALL. I got teary just reading your request.
All I can suggest is that you enjoy the time you DO have. Try not to stress about the little things. Your children won't really care or remember whether your house is perfectly clean, or whatever. Enjoy the weekends! Revel in the time you have! Be in the moment as much as you can muster.
And consider this too; I grew up in the seventies when almost everyone's mom was home full-time -- and yet our moms rarely played with us. It just wasn't done. Kids ran around and played with each other. Moms were in the background cooking and cleaning or doing mom-things.
My point is that there is an EXCELLENT chance that you are giving your child more "real" time with you than many mothers who do stay home. And you are appreciating it more.
I've done both. I was a stay-home mom for years, and then I went back to work and was on the road shooting films -- please believe me that wherever you are, the grass is ALWAYS greener on the other side.
I actually had the opposite ephiphany, which is why I went back to work when my daughter hit first grade. Not that I didn't appreciate it, and enjoy it, BUT...I was going CRAZY being home full-time. Then I switched back to SHM when she turned 10 to enjoy the few remaining years before she thinks I'm a total loser. (well, I'm already pretty uncool, but still).
YOU are his mom. No one else. Nothing can replace a mom. Think up special silly things that he'll always remember, rituals and so forth that he will associate with you forever. For instance, pick a special date (not his birthday) and call it "Josh Day" (or whatever his name is). Do a special activity with just him that day every year. No dad, no brother, just him. When he's older he can choose the activity, but for now, make it a picnic at scenic spot, or something else that will be memorable. When he loses a tooth, get some sparkly nail polish and leave itty bitty "footprints" on his pillow case and then paint a silver dollar with the "fairy dust". Take two minutes to draw little cartoons or notes to put in his lunchbox every day...There are so many things that he will remember and appreciate later on...and more importantly that NO ONE else can/will do for him.
You don't have to be there every minute to be the most magical, important presense in his life. I know this doesn't help with the "missing"...nothing does, but perspective helps. You love him more than anyone. That's a fact. And there are a million little ways to remind him.
If you can find a different job with better hours, great. If not, you can still be the Best Mom in the World. And to help with the "missing" have you considered giving your MIL a video camera? Granted, it's not "the same" as being there...but kids love watching movies of themselves, and sitting down with your son to watch some of his activities on film could be really cool for both of you.
Well, I am rambling, but I hope this helps even a tiny bit. Best luck!