If you believe that the environment in which he is residing during visitation poses a threat to his well being, you should Petition the court to modify the PP. You may ask for a GAL, but be warned, you will be expected to pay for the GAL -- there retainers can run anywhere form 2000 to 3000.
The information is for general information purposes only. Nothing stated above should be taken as legal advice for any individual case or situation.
Check out my AVVO Legal Guides on Modification of a Parenting Plan. Leaving the 9 year old alone may violate state law. You can make a referral to CPS, or you could even think about calling the police if your sone is left alone too long.
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It is a good idea to seek a Modification sooner than later should you have a concern. You can ask the court to appoint a GAL so that your son's voice can be heard and best interest advocated for. Its a good idea to seek legal advice in preparing and filing for the Modification and appointment of a GAL.