My son has came first and does come first
Sorry OP but you chose to have a baby with someone you knew was a drug dealer. Then married the drug dealer. Then stayed with the drug dealer when he turned out to be a drug addict too. Then continued to offer the supposedly ex dealer and supposedly ex addict emotional support during his supposed attempts to get clean...
None of that was putting your son first.
It sounds as though the growth of your emotional maturity perhaps ended when you met this guy, so while you're now late 20s (I'm guessing), you're making the kinds of decisions that someone in their late teens with limited life experience and lots of naivety would make.
And that would be fine I guess, it would be your prerogative to take that risk, if you didn't have a child who needs to come first.
If he can't get through rehab without contact then he isn't in the right headspace to get through rehab and he will fall at the first hurdle.
Because this current arrangement, where you aren't 'together' but he is leaning on your emotionally, is setting up a dynamic where he has made you partially responsible for his wellbeing and potential sobriety.
You know what that means? It means years of "if you don't let me come and see you and (son) I'll just go out and get coked up because I'll be so sad" / "if you loved me you'd let me have (son) this weekend but clearly you want me to fail so fine I'll go out and get wasted, happy now?!" / "you clearly don't care if we can't spend (xmas, birthday, whatever) as a family so I may as well just kill myself" etc.
Emotional manipulation. You're being manipulated right now and set up to be manipulated in future too.
When you said he needed to get clean what did he do? He got his mate to do the urine tests so you wouldn't know he took drugs and he went out and saw friends, had a laugh. All your words, not mine. Hardly the actions of a devastated man desperate to save his family.
Some things aren't fixable. Having a baby with a drug dealer is something that cannot be fixed and needs to now be managed. The way to manage for now it is to have no contact with him while he's in rehab and to spend that time yourself having therapy. Your therapist can talk you through what your next options are but right now, contact with this man is not in your son's best interests as it's taking up his mum's time and energy while also maintaining his mums bond with a figure who has put him at risk since the day he was born.
It's time to grow up I'm afraid. You aren't that young girl anymore. You need to take responsibility for the situation you and your son are in and put your son first from today on.