I'm willing to put money on the fact that your husband came from a family of screamers. My husband used to do the same thing - flip out in situations where calm was needed because that's what he learned by example from his parents. I didn't fully realize it until I witnessed his mom (my MIL) flip out on my husband when his dad had a diabetic incident. Somehow the fact that my FIL was on his hands and knees on the floor in a diabetic situation became my husband's fault... ??? They jsut didn't know how to handle stuff so they screamed - I bet their parents did when they were kids too. Poor coping skills.
You are 1,000% right that your son yells at you becuase he sees dad yell at you and thinks this is how we communicate with mom when we're frustrated. Kids learn more by what's "caught" than by what's taught. The expression "do as I say not as I do" is as old as the hills - where does he think it came from?
It will take a lot of heart to heart conversations when it's calm to get your husband to truly understand. You have to give examples from your new little family, and if possible form the family he grew up in. In his heart he knows that it's true - but doesn't want to be at fault. Who does? So you have to talk about it in a very non-accusatory way. I suggest that in the process you also 'fess up to your parenting shortcomings as well so your husband sees that you're not just attacking him, but you're evaluating how the two of you are parenting your child and how you can improve it.
Begin by saying how much you love your son. Admit that you feel a tremendous sense of responsbility to do the best job you can possible do to get your son to become a kind, honorable, strong man of character. Tell him that you want the two of you to be a team and become the best parents possible. Ask him for suggestions for how both of you can "improve your game". See what he says. Do not jump down his throat. pause and really think about what he says. Don't be surprised if he says he thinks he's doing just fine. There's something about a man's psyche that makes it very difficult for them to admit that they could possibly be at fault in any way.
Tell him how much it hurts to be yelled at. Don't use the words "always" or "never" - just explain how hurtful it is. Ask him if he can understand that. Chances are he can becuase he was yelled at by his parents. Tell him you want to do everything possible to raise you son in an environment where yelling is not the norm in stressful situations. Suggest that you think your son needs to learn better coping skills so he can better handle difficult situations.
Ask him what can we do instead of yell when we're stressed? Can we count to 10 (or 100 if needed?), can we leave the room temporarily? Can we jump up and down until the desire to scream leaves us? Maybe you two can come up with a ridiculous code word to use instead of yelling that has the potential to make you both laugh instead? "pogo stick", "motorcycle", "ceramic tiles" or "chocolate chip cookies"?
Assuming he agrees to try, expect that there will be times that the yelling will stlll happen. I have to say that it wasn't until my husband was in a serious auto accident where he nearly died that he changed his approach. He realized that we were ready to send him to live in the nursing home next to his mother if he kept it up. He also realized what's really precious to him - his family. I think he also got a good look at all the stuff a mom does since he really couldn't do anything other than sit in the recliner in the living room for a few weeks and watch family life go by. He couldn't return to work for almost 5 months and so he got a realy eye-opener on what family life really entails.
The best thing you can do for this situation (aside from getting your husband in an auto accident) is be away from your home for a few days so he can learn what it's really like to be a mom/homemaker. It's an eyeopener.
Good luck mama - husbands do seem to mellow as they get older and they become better dads - but it does seem to take a while and lots of training! ;o) Too bad they don't come fully trained.