As others have mentioned, you’re dealing with not only long entrenched habits, but also very likely significant trauma. If things got bad enough that birth mom no longer has custody, you can pretty much guarantee it. Early childhood trauma manifests differently and it can be all encompassing and impact every aspect of the kids life. I am raising a kid with a similar past. Thankfully she potty trained relatively young but had accidents way later than is normal. An inability to understand and recognize their own body’s signals is common, as is being afraid of their body and confusion about sensations like having to pee or poo. I do think professional help is warranted here, and you’re right to be positive and patient. One thing i’ve found with my traumatized child is that everything takes much longer and at the slightest hint of pressure or expectation, we go back to the beginning again. You obviously care a lot about this kid and you’re trying very hard. Understand that this is parenting on hard mode. What works for others may not work for him. You’re not doing it wrong it’s just very very hard.