ANSWER:

Once he’s an adult, he’s responsible for his own actions and will be sinful and parents are also sinful for not raising the kids on islam. 

QUESTION: If the parents raise up their children really badly. Don’t force them to pray or fast. Themselves don’t pray. Don’t know about Islam send their children to secular coed schools. If the child then grown up and does not pray is completely worldly and materialistic will he still get sins or will his parents bare his sins for raising him up without teaching him Islam.