Just to present another POV - your DN is only 24, he’s a care leaver, he has some history of domestic violence and possibly some other issues in his past given the fact he only has contact in a centre.
You’ve said yourself he would need support to parent. 24 is young to be a parent especially for someone who wasn’t properly parented themselves. It’s possible if his son was placed with him that placement wouldn’t work out.
Do you think it might be in the best interests of the baby to be put up for adoption now? He’s already had a rough start in life, he’s still young, so there should be time to make up for that, but it there is a long dispute about his care, his father has him for a bit, that doesn’t work out, he gets taken back into care.. at this point he will be more disadvantaged and finding him a good adoption family will be harder.
Just to be clear, I think it’s really important parents who need help are supported to keep their children, but this has been tried with the mother and it hasn’t worked. Given your DN needs support it may very well not work out with him either.
I understand you want to keep your great nephew in the family but what is best for him needs to be paramount. It’s also possible that it would be better for your nephew to focus on building his life skills and experience, and leave parenting until he’s in his 30s and has a partner.
I just worry that your great nephew is not having good start in life at all.