K., it's not your place to diagnose him. But to play along for a bit...
There are different forms of bi-polar depression. Some people cycle rapidly and severely while others find that their symptoms come and go over long periods of time. "Manic" phases can include things on the milder end of things like impulsive behavior (sexual impulsiveness, spending money, shopping sprees, quitting a job), lots of energy and little need for sleep, behavior that looks like "speeding," to things like drinking or drug binges, disappearing for days (road trips etc.) all the way to full-on psychosis. "Depressive" phases can include milder symptoms like fatigue, irritability, wanting to sleep a lot, feeling unmotivated, feeling anti-social, not wanting to spend time doing things that normally bring enjoyment or spend time with loved ones all the way up to debilitating things like not being able to get out of bed, not being able to work or care for oneself or others, being in physical pain, being suicidal. Some people with bi-polar depression cycle from one side to the other while others have periods of normal behavior and still others experience both at once. Generally, manic phases are shorter than depressive phases but some people spend time equally in both.
I can tell you that it took my husband almost 20 years to get diagnosed with bi-polar. He was repeatedly diagnosed with and treated for depression with ADHD. It really wasn't until we were married and was able to share my observations about the cyclical nature of his moods that his doctors were able to see the bi-polar and treat for that. Even my husband didn't recognize the cycles, and it's his body and brain and life!
At the end of the day, it doesn't matter why he behaves the way he does. You are not safe, your child is not safe, your husband is a ticking time bomb that's not being treated.
Please get you and your son safe before considering what could be behind your husband's behavior. He may have something treatable, or he could just be a violent SOB who will never change. Figuring out what, if anything, is wrong with him and treatable will take a long time and you don't have that luxury. I understand wanting to save your marriage, but you've got to save yourself and your son first and worry about your husband second.
FWIW, my husband isn't violent. In 10 years of marriage he was aggressive towards me once and I had him arrested for it. Mental illness doesn't excuse violence.