does he love me or is he just saying this because he knows I still love him?

Need to know your ex's personality to really tell you the answer, and is he in jail for little things or big? Is he in state Prison?

If he's a nice/no games guy then yea he's jealous, he still love you or at least his love for you starting to raise again. When a man all alone, they will think about their most loved person, in this case which is you since your his ex. The more he think about it everyday because of nothing to do, his love for you will raise.

If he's a mind game person then you might need to step back and see what he wants, some times prisoner(s) force other prisoner(weaker ones) to do stuff like drugs, money, porno etc. If he's been controlling by the tougher prisoner(s) or even gang inside he could be trying to get your trust and then pop the question like bring him drugs, money etc.

You need to decide if he's a good or bad person overall(how he treat you, people, friends, family), not just how he treat you now.