I think you need your own lawyer to represent you in this matter, especially if you are not comfortable with the efforts of his attorney. Double-teaming the opposition works well. If you've had the child that long, you've got standing to seek his conservatorship. HIre an attorney to file a SAPCR modification for you seeking custody.
If you're in Belton, you could talk to either Carol Prater or Pat Ridley. I know them both. They are execellent attorneys.
I am not intending this to be legal advice, because I don't know the particulars of your situation. Call me if you would like to discuss this or other isues.
An intervention is a filing into an ongoing lawsuit. In this case the intervention would be into a custody case. You need to speak to your son about the long term goals for the child, and if it is time for you to hire your own attorney to fight for custody of the child.
Your own attorney can best advise you on about the strength of your case and how to best accomplish getting custody of the child. In Texas the calculation of the time the child spent in your care will be essential in overcoming the “parental presumption” in favor of the biological parents of the child.