It appears that he cannot do it on his own, but Congress can.
I'm not really sure whether dismantling the Ninth Circuit is a good idea or not. I'm not sure how it is determined how much area a circuit covers and whether the Ninth is too big. However, I do think this is a retaliatory move on Trump's part for them knocking down his deportation lawsuits and not something that would otherwise be on his agenda.
That may be so, but that would never have entered Trump's orbit if it wasn't the circuit that knocked down Trump's deportation lawsuits. If they had agreed with him, there would be no talk of dismantling or splitting up the circuit.