I don't have much advice, I just wanted to say that I really respect how you're sticking to the boundaries you set for yourself, and recognizing that he's not really working his recovery or this opportunity for all it's worth.
Can you imagine how happy you would have been to get to spend 14 months in a program to overcome your addiction? This is truly a blessing, it's sad that he doesn't appreciate it.
Just keep on keepin' on. As long as you always put your son and you first, things will work out however they are meant to. When my mom starts down the "you're not supportive" road, I politely end the conversation or divert. That sort of engagement doesn't contribute to my recovery or my life in a healthy way, so it's not welcome at this time. Same with you: if his parents say things that you infer as judgement, then it really doesn't matter their intention, it's not always in your best interest to continue participating in that line of conversation.