How long have you been married?
Have you tried ever expressing your feelings to him at all?
Unfortunately, that IS the first step. To see where your husband stands.
I use to try to bottle is up, and it all eventually would just come out in rage. which isn't productive at all, and THATS when you look like a bitch.
But use "I" statements.. I feel, etc.
Don't use "you" statements...Even though you want to, try not to accuse your husband or anything or his kid bc that will just make him instantly defensive.
How often do you two have the kid?

Ummm, first of all... Your husband needs to step up not you. He needs to define the boundaries in the relationship between he/you and his son. You have to understand that this boy probably misses his dad and wants as much time with him as he can. If he only sees him 2 or 3 days a week max, he has has a void there. Fathers and sons have a special bond. The boy is afraid of losing the only time he has with his dad. When the boy is there, maybe you can try to let him have his time with his dad. This means, let him snuggle up to his dad and you lay low. You have his dad all the other days when he is not there. This will hopefully lessen his need to be with him at night or on off hours. If it doesn't then your husband and the boy need to have a talk about you and the need to share both he and you. Your husband needs to tell the boy how important you are to him and also how important he is to you. I think the boy acts like this because he sees you as a threat. Given that knowledge, be kind. You are older and wiser than him and you are the adult here. Make him see that you are not a threat and not taking his dad away. He will likely warm up to you and the idea of you and his father over time. Lastly, bottling up these feelings does not help anyone. You need to express your feelings but do it with care. I am a stepmom, the girls all smothered their dad at first and I let them. After a while they were curious about me and now they smother me lol.

Good luck.