Tell him he can invent his own table for the Rod.
In one of my games we love Wild Magic. When I GM I make my own table and regularly replace entries or move them around. Things like the Rod of Wonder and Wild Magic should be completely unpredictable IMO.
I had a wizard once who cast a spell and had a wild result. He woke up in a forest glade covered in fatty, grey tissue and blood. Next to him was an altar with figure from a race he'd never seen before. The back of it's skull looked as if something had exploded out of it.
Surrounding the altar in the glade were 7 or 8 stone monoliths that if you squinted real hard, kind of looked like owls. I say 7 or 8, because regardless of any attempts to count them, you always got a result of 7. Or 8. Or maybe it was 7.
He left the glade and found himself in a lushly carpeted valley. It took him some time to walk out of it, he maybe traveled 5 miles, and he wasn't at the center. When he got to the lip of the valley he looked back and it only looked to be a mile across. Below him on the other side of the hill was a farmer looking in stunned disbelief. The farmer was pointing at the valley and talking about how that's where his farm was. That the valley hadn't been there this morning when he left for the village. I should point out that the region surrounding this valley was a desert (where the campaign was taking place).
Eventually the wizard made it back to the city where the party was. After a few days rest he mysteriously found one of his highest level spell slots was replaced by a spell called "Owls". When he cast it his body fell over unconscious and he dreamed about being back in the glade with the owl-like statues. Unbidden, a thought entered his mind that it might feel nice to smash his skull against the alter.
A week later, a second spell slot was occupied by "Owls".