If his actions once he gets out don't match his words from jail then I would chalk it up to jail talk alot of men when they go to jail say things like that so that their girlfriends whom they didn't treat very well while out on the streets won't dessert them when they need them the most and he knows that is what you have wanted to hear from him and he knew that if he told you that you would wait it out with him and stick by him. If when he gets out his actions do not match what he said in jail I would walk away and never look back you deserve better than that.
I would wait and see how he acts towards you when he gets out (if he hasn't already) and if he acts the same as his words then he meant it but him he gets out and it's the same thing different day then I would question him on why he said those things while he was in jail. If none of the other girls were there for him when he went in then I would be inclined to think it was because of gratitude that you stayed or maybe he did have a revelation when in jail and realized his feelings for you. Only time will tell!!!!