Hi! That's really sad. We got a taste of it yesterday while my husband was away on business.
I've heard that, if your little one can visit Daddy where he works, get a little "tour" and see what Daddy does when he's away, that really helps. That way, later, you can talk about what he might be doing at any given point when your little one is missing him. Also, if your husband has a desk or a locker or some place where he can store personal items, make sure he has a family picture taped up or framed, and have your husband point it out, so your son can see that Daddy is still thinking of him when they're apart.
You might even get related toys. Like, if your husband works a forlift, or whatever, you can find something similar and play with your son, and he can pretend he's at work with his dad.
Can your husband make a call every day when he's at work, before the anticipated most-upset time? (I don't think it's a good idea to wait till your son is *really* upset - catch it before then.)
Of course, when his dad IS home, make sure they're getting as much good, quality time together as possible. Have the TV off, keep that newspaper off his lap and make sure Daddy is filling up that love bank. This will actually make the times apart much easier, not worse. Same with you. Give him as much love and security and good together time as you can. Lots of eye contact. Lots of good listening and paraphrasing (to let him know that you're listening). Lots of good hugs. When you hold his hand, hold it tightly (but not too tightly!). Remind him that Daddy always comes back.
Don't try to make him be a little man. He's still a baby. If you try to make him independent and "suck it up and get used to it" before he's ready, he'll just be clingier. Reasoning and telling him Daddy's earning money for the family won't work. But if you respond to his needs - hopefully even before he gets clingy on any given day - it will ease up on you and he will feel more secure and happier all around. It really works!
Hang in there.