Your son is receiving services but have you looked at New York State Office for People with Developmental Disabilities (OPWDD)?
And have you ever checked out YAI?
New York State and many local governments provide support and assistance for persons with Developmental Disabilities. You had to work your way through the system to obtain the SSI. Someone you met in that effort could surely let you know if your son can receive SSI and PA at the same time.
The answer provided to you is in the nature of general information. The general proposition being that you should try to avoid a bad outcome if you can.