We stopped it by having him earn screentime through non-complaining compliance. He needed to do what was asked without complaining or else he didn't earn his reward.
With the toileting work we've been doing over the last few months, Alex started earning a lot of rewards regardless of how well or poorly he complied. This has led to a new outbreak of complaining. Once again, we are subjected to constant "no
I could say to Dave: It's sunny out.
Alex: No sunny out.
It's not socially appropriate behaviour and so we've got to work on it. Since we've removed screentime as a reward for toileting, it's getting tied to compliance again. Things will get worse for awhile as he tries to get us to give up on our standards, but hopefully he will come around again. Meanwhile, we are dealing with a lot of tantrums.