You need to look at what is in his best interest, not what is in yours. If he's well bonded with the foster family, he needs to stay with them. It's likely that you will lose in court, despite being a relative. The judge often rules based on the recommendation of the social worker of the child and what they feel is in the best interest of the child.
What interests you in this boy if you didn't even know he existed prior?
Foster is also slated to be temporary. He may or may not become eligible for adoption at some point. Parents being in jail doesn't mean they won't regain custody at some point.
If his parents will be out of jail in a few months and then complete their parenting plan, they will get him back meaning the child would have to endure another move and transition to another home. This can be detrimental to his well being, being bounced around like that. The goal in foster care is to be with them, not you.
DNA makes you related, it doesn't necessarily make you family.
Being related doesn't make you the best option either.