Vent away! It seems that we have some things in common...my 14 year old daughter (husband's from first marriage) feels the need to lie & hide things (like a boyfriend for 6 months), and my spouse is (in my opinion) far too forgiving. I have a couple of posts about these very issues and it seems that I'm bearing the brunt for being the step-mom and too suspicious.
What is the relationship with his mother? Does your stepson live with you most of the time? How long have you been in this boy's life? I know some people will not agree with me, but by virtue of marriage, you are a parent to this child. As such, you have a right to discipline inappropriate behavior when he is under your care.
It seems to me that there are multiple issues at play:
Respect, discipline, guilt complex, and testing his boundaries. While some of this stems from being a teenager, some stems from family situation. He may be angry that his parents are not together anymore. He may believe that he can get away with anything because his father feels guilty. He may feel that he is betraying his mother if he appreciates anything you do for him. He may not get as much from his mother and feels that he has to take things to get things. He may have learned that attention for bad behavior is as good as the opposite. He may just be a rude and unappreciative individual. (Sorry if I seem harsh, but it could be the reality).
The lying, stealing (temporary borrowing?), and loose punishment must stop. You and your husband need to discuss discipline for all of your children and how unified you two need to be. Your husband should stand up to his son, no matter the consequences, and nip this in the bud NOW. You are parents, not a recreational stop.
Our daughter has taken to spending less time with us (partially due to friends, but I suspect some of it is due to our attitudes about her life). Although I would like to see our daughter more, I won't do it at the expense of compromising discipline in favor of her perceived happiness. If she thinks I'm a pain for the next 5 years (until she graduates high school and gets out from under her mother's thumb), then that is a title I am willing to bear. A respectful child is more pleasant to be around than an undisciplined one.
I wish you the best of luck with this. It is not a pleasant situation and it is just more amplified when you feel helpless. The son needs to develop respect for you now so the remaining childhood years will be more enjoyable for all.