My ex led me down this path so I speak from experience. I am sorry that you are facing what you are facing. And I'm sorry your son has been exposed to addiction and loss at an early age.
I know as long as I stay in the present and don't "future trip" and as long as I put the best interests of my child first I will not make choices that I regret. So I try to be very cautious when it comes to trusting an addict. Fool me once shame on you, fool me twice shame on me....
I just want to get back to my "normal", back to before things went so horribly wrong.
Unfortunately, I think you are going to have to face a "new normal". Your fiance is in prison for B&E. He has stolen from you. And he is addicted to drugs. Most addicts would be great people if they weren't addicts, but the problem is that they ARE addicts - and if they are already so far into the drugs and the life style that they are stealing to support their habit - they are pretty far along into their addiction and change is REALLY hard to sustain. Oh sure, it's really easy to talk about, but it's really hard to actually make it happen. So I'd really be carefully about believing any promises that an addict makes from jail. He may even believe them at the time he makes them but the proof is in the pudding - it's what he DOES when he gets out that matters. It's easy to say I'm changing my life when you are incarcerated and not faced with the difficulties and challenges and choices that are available in the real world. And of course, a person in jail is going to say anything he can to convince you that he has changed so he doesn't want to lose the life he had before he went to jail.
I encourage you to read everything on this website, focus on your son's future, let your boyfriend do the time for his crime while you build the best life for yourself and your child that is not dependent on someone who would be a really great guy IF he wasn't in jail or addicted to drugs (cuz that's a huge IF).
You have no control over what your boyfriend does when he gets out of jail. But you can have a good life no matter what he chooses, as long as you make wise choices.