A Problem With Zen, He bites Options

[Chaos, my little...]

As I have posted three weeks ago, I adopted a Siamese from a local shelter. He is a two year-old male whose "owner" dropped him off at the shelter after having him for two years. He allowed Zen (formerly known as Monkey) to go outside quite often, but I suspect that when he was indoors he was kept in a cage. Well, he was at the shelter two months when I adopted him, so that was traumatizing in and of itself.

He is VERY loving and enjoys being around me all the time. He sleeps with me (and boy does he snore!), with his paw oftentimes draped over my arm. He is adorable, and except for one thing, I have no concerns about him other than his weight. The problem is that in the past three weeks since I have had him, he has bitten me four times. Not the play bites either. He didnt break my skin, but he definitely bit me hard (twice I had to pull him off of me by the scruff of the back of his neck as he not only wasn't letting go, but was trying to get a better grip on me). The biting happened (3 out of 4 times) when we were laying together and I was simply stroking him (we were not playing). There was very little warning before he decided to bite.

I am not giving him up, as he is a wonderful animal, and I really do suspect that he was abused by his former "owner" (I really hate that word) due to some other things he has done (when I ask him to leave the plant room, he is stubborn, and when I go to usher him out with my hands, he runs away in a very anxious manner while crying in a frightened way-that is just one example). What I want to know is whether or not anyone has experienced this problem with a feline, and what I can do to show him that I make the rules, and biting like that is NOT allowed, while still acting in a loving manner. Thanks in advance for the advice.

-Jenn