As a parent to two step children, my partner - now wife - and i agreed from the start to back each other in front of them, and sort out any differences out of their hearing.
This is the only way, or else any child will play one parent off against the other. It's no comfort, but the same applies to 'bioligical' offspring as well - it's what children do.
You need to have a calm talk with your husband in private. Explain that loving your son - which he effectively is - is hard enough, without the 'divide and conquer' situation you are all in - which is making all of your miserable.
Your husband feels guilty disciplining his son, and you are a convenient scapegoat - if he makes you the bad guy, his son need never think badly of his dad.
That's your husband's subconcious thinking, but it is flawed, and he will not win with it.
Teenagers need consistent boundaries, laid down and applied. It makes them feel secure, even though their instinct is to try and break through, and that's part of growing up.
You must get your husband on side, you must back each other - right or wrong, and be united. Couple that with some 'one to one' time for you and your husband, and each of you and your son, and you can forge some links instead of settling down for a war of attrition.
It takes time, but if your husband will not back you, you should seriously consider your future in this relationship.