I work in the bankruptcy field and can sympathize with your situation. As a creditor, you have the right to show up at the hearing (Meeting of Creditors) at the date and time shown on the paperwork that you received. However, in a Chapter 7, there are rarely any assets to be liquidated. If there are, you will receive a separate notice from the bankruptcy court indicating that assets are available and you would need to file a proof of claim to have access to the money (distributed pro rata between all creditors who also file a proof of claim).
One thing that you can do is review a copy of the bankruptcy petition (they are public record with the exception of a few pages) against a copy of a recent financial statement to look for discrepancies. For instance, if he claims to own a vehicle on his financial statement but it isn't listed on the bankruptcy, you can call the bankruptcy fraud hotline and report him. Bankruptcy fraud is VERY seriously taken by the court and examples are made of the people who are caught. Additionally, if he lists far less income on his bankruptcy petition than on his financial statement, you could file an objection to the financial statement based on his petition (which is signed under penalty of perjury).
Finally, as you are preparing to litigate the custody issue, you can bring up the fact that you are "concerned" regarding his ability to monetarily care for your son as evidenced by his recent bankruptcy filing. Don't get too "crazy" about it, since those two things truly are not linked. To be honest, he technically has a "benefit" since although his income is low, he now has no bills.
I also wanted to mention that I recently went through a custody battle for my 13 yr old daughter and I did raise the financial concerns issue since he had not paid child support in a long time. It was one sentence in my otherwise lengthy memorandum, but it was taken as a factor in the overall decision. However, I would let your attorney make that final decision.