@bmh3255,
As others have noted, you should find an attorney to advise you on the legal issues. The following links may be helpful.
National Alliance on Mental Illness Legal Support
http://www.nami.org/Content/NavigationMenu/Find_Support/Legal_Support/Legal_Resources.htm
National Organization of Social Security Claimants' Representatives
http://www.nosscr.org/
Based on your post, it doesn't seem that there is any active legal issue or dispute before a court regarding the custody of your son and thus you would only require minimal legal services. You should be able to get a referral to an attorney who can provide a free or low-cost consultation.
However, it's very unlikely that you would lose custody of your child solely as a result of filing a Social Security Disability Insurance program claim. If there were a legal issue or dispute regarding the custody of your son, having a reliable source of income such as disability insurance payments would likely be a point in your favor.
In most jurisdictions in the United States, psychologists and psychiatrists are legally required to report credible suspicions of child abuse and/or potential harm to a child to law enforcement or child protection agencies even if their suspicion or belief is based on communications that would otherwise be confidential and privileged. Assuming you are getting treatment for PTSD and doing reasonably well, you should have nothing to worry about as if your condition posed an unacceptable risk to your son, it's likely this would already have been reported and you would know about it.