I was in the military for 10 years and had to leave my daughter with other people from the beginning (she's 5 now). I found that the best thing to do, even as an infant, is to #1 get his attention (sometimes I had to hold my daughter to get her attention),
#2 tell him that you am leaving and what you will be doing (taking out the trash, going to work, shopping, going out with a friend, etc),
#3 how long you will be gone (I'll be right back, I fogot something in the car that I have to get so I'll be right back but I'm going to have to leave again for work, I'll be gone for a few hours, days, weeks, months, long time),
#4 assure him that you will be back,
and #5 let him know your back as soon as you get there.
If he has a big problem (which my daughter would every once in awhile) you'll want to work it in baby steps. Start with taking out the trash and going to the bathroom, or just being in the other room. At 8 months I'm sure he can crawl pretty good by now so you might put him in a room, get his attention and let him know you'll be in the other room if he needs you. Then he can crawl to you. If he starts crying or screaming for you call to him and tell him to come to you.
Then you can put him in the crib, say you'll be in the other room to get something (treat for him like a teething cookie, toy, or something) and will be right back. I'm sure he'll have a fit, which is fine, but when you come back tell him "What's the matter? I told you I would be right back. I just had to go and get this" and show him what you went to get. If it's a treat or a favorite toy he'll be much more understanding and happy if you have to leave to get him something that he really likes.
What this is doing is building his trust. I'm sure that your husband tells you where he's going or at least that things are routine enough that you know what time he's most likely to leave for work or come back. But, if you looked up and he was gone without letting you know that he was leaving you'd start to wonder why he left, where he went to, etc. And if he didn't come home around the time that he normally does or if he took 4 hours on a grocery store trip that should have only taken at most 1 hour you would worry, call him if he has a cell phone, etc. Your son feels the same way. If you take off for an evening with your husband and sneak out while your son is distracted then how is he going to trust that something bad didn't happen or that you'll even come back? He screams because it's the only way he knows he'll definitely get a hold of you and get you there fast.
If you take off and don't tell him where you're going he's going to be afraid that as soon as he's not paying attention you're going to disappear on him. It will take a little while before he understands what time is but he'll pick the concept up faster than you may think.
Building up that trust is going to take awhile, lots of patience, and tons of practice. When I was getting ready to leave for Iraq I knew I was going to be gone for at least 6 months. So, what I did was go to do some training I knew I was going to have to do sometime anyways. It was about a month and a half long and I would be gone all week and back on the weekends that I could afford to leave. I did it so that she would understand that sometimes I had to be gone for awhile but that I would be back. I bought her 2 calendars. One for her room and one that she could carry with her. The one that she carried had a picture of our family that she could look at and carry. Inside she would cross the days off and it showed what month I would be back. This helped her with coping while I had to be gone. I also made a video of me reading a book and she was able to watch the video and flip through the pages by herself whenever she wanted. Thank goodness I don't have to worry about doing that again but, because I had gotten her used to me telling her that I was leaving, where I was going, what I was doing, and that I would be back, she had that trust built up.
She's 5 now and I still make sure to tell her whenever I go outside, and what I'm doing. There was one time that I was cleaning out the car and it was taking longer than she thought it should so every once in awhile she'd pop her head out the front door, call for me, and ask what I was doing. Then she would feel better and go back to what she was doing. Sometimes, even now, she'll start pouting and crying if I tell her I'm leaving for somewhere but I'm always very firm about it and once I know she understands what I've told her I leave. When I come back I make sure to let her know I'm back.
The main thing I hope you take with you is to build that trust by:
#1-Get his attention
#2-Tell him, under no uncertain terms, that you're leaving
#3-Tell him where you're going
#4-Tell him what you will be doing while you're gone
#5-Give him the length of time that you will be gone (You can use an analog clock-meaning one with hour & minute hands) and show him that you will be back before this hand gets to here. You might even get one of the little signs that has a clock saying "Will be back at..." and put it next to the analog clock whenever you are leaving for somewhere. That way he doesn't have to remember what time he can reference the other one and match it up.
#6-Let him know you're back
It might take a few times of telling him the same thing to make sure he heard everything you said. His comprehension will go up at time goes on.
You will get a lot of resistance at first but after awhile he will get the concept and that trust will be built.
I know this was super long but I really hope it helps.