Okay, as a cancer survivor I was prescribed 5mg of oxycodone at one point too, and you have to be in a LOT of pain to get those kinds of meds (as in, my bones were collapsing in on themselves; I can imagine what you’re going through). If your husband was stealing much-needed prescription pain meds from his cancer patient wife, and also had a two year affair, I would suggest with all the kindness in the world that whether or not you want to take him back is the very least of the issues here.
He’s an addict, a cheater, he’s highly unstable, and he has sought no treatment for any of his problems. Before he’s capable of having a meaningful, healthy relationship he’ll need to work on his issues for a sustained period of time, and demonstrate that he’s stable for an even longer period afterwards. And he will need to do so without an enabler. He’s put you in the position of enabler by stealing your meds, cheating, lying, etc. and having no actual repercussions for his actions. Why do you keep disrespecting your own boundaries for this man? You’re unwittingly teaching him how to manipulate you, and it appears to be the only thing he’s learned from his mistakes.
You need to realize that your entire cancer treatment and life or death could very well end up devoted to, and revolving around, his issues, not yours. Is that what you want? Understand that you’re feeling sorry for him because he’s struggling: now imagine how he ought to feel about you, going through actual cancer. But he doesn’t, does he? Why isn’t he there for you when you’re struggling? Why does he have you always helping him, while he’s hurting you? Your son appears to be the only one who feels as he ought for you, but you reject his loving concern in favor of poor treatment from your husband. May I ask why? Your son is supporting your stated boundaries, and your husband is asking you to trample them. May I ask what you’re teaching your son about relationships?
Your son is correct in pointing out that your relationship with this man is unhealthy, the balance of power is massively unequal and the dynamic is toxic, and that taking him back would amount to enabling behavior. Hypothetically, what if you took him back and he were to overdose on your pills?
Have a serious think about why you’re with this man. When I was sick, a partner’s problems could have provided a distraction from, or seemed easier to face and fix than, my own serious health issues. I’m just being honest here, cancer is so freaking hard. Just know that you matter, too. Maybe have some therapy and focus on yourself. You’re going through a LOT, even without this man.
If it were me, I would leave. The stress of this could be very bad for you. I’m so sorry you’re going through all this.