Potential PDA/Autism 10+ Y.O Child

Parent here of a child that we suspect has autism ; awaiting a diagnosis however the evaluation is 6+ months out due to specialists in the area being oversaturated with patients and such.
Currently our child has always struggled with transitions in all walks of life, exhibiting many of the symptoms of PDA. Consistent meltdowns when proposed with the requirement of making a decision or choice. We thought this was more behavioral in nature for a while, however simply seems he has anxiety with the concept of making a decision or responding to a command. High level child in other areas however is easily over stimulated, anxious and has a difficult time with listening to instructions due to this.

Anyone have any strategies that has helped transition a child with suspected PDA/autism that can better him listening or making decisions versus shutting down?

I have been giving him choices and telling him up front the results (both good and bad) between choice A and B, and letting him choose on his own time versus him listening to a command.

I am his biological father, he also has dyslexia and ADHD additionally if this helps.

Thanks in advance for your time!