Nico's healing abilities only work if it's necessary, such as when he halted his own blood flow to save his sister from Maxie and Giratina. Because of this, once Doom brings him home, Nico will remain comatose until something happens to Doom and Tempora (probably due to either Maxie or Damian), which then causes his healing to kick in and wake him from his coma. Did that help?