You can only submit a claim for the $1200 in small claims court IF the debt was incurred after the bankruptcy case was filed. (referred to as post-petition) If the debt was incurred pre-petition (before the case was filed) then it will be subject to discharge in your step-son's case.
Now you can attempt to proceed against him and attempt to make the argument that the $1200 was not listed in the bankrutpcy case, but you may cause more harm than good to yourself. Proceeding against your step-son for debt that is included in his bankruptcy case will be considered a violation of the automatic stay he received once the bankruptcy case was filed. Bankruptcy Court is a Federal Court and to violate bankruptcy law will be violating federal law and there are penalties for violating the automatice stay.
You should consult a bankruptcy attorney in your area, if you truly want to look into proceeding against your step-son for the $1200. Someone needs to analyze the specifics of your situation and determine whether attempting to collect the $1200 would be benenficial and/or legal.
On first impression, it would appear that it was your step-son's intent to include what he believed to be the total amount of debt owed to you and/or his mother as of the date of filing the petition. Furthermore, if you wanted to pursue the claim while the bankruptcy case is open, you will have to file it in bankruptcy court or request permission from the bankrutpcy court to proceed against your step-son in the small claims court. The test is when the debt was incurred. Only debt incurred before the case was filed will be discharged. Your step-son will be responsible for any debts incurred post-petition. Therefore, if the $1200 was incurred post-petition, then you can try to collect. However, if it was incurred pre-petition, it will be discharged in the chapter 7 case.