The dew claw can be sore if it catches on something like getting your finger pulled back.
You can see the quick in the first of the other claws too - it ends at the tip of the black line so there's only a couple of mm you could clip off that one if that. And I imagine it would be about the same for the dew claw too ie if you clip more than 1.5mm off it would hurt and bleed. And good luck persuading him to let you try again.
I'd go for clickering and nail file any time you have some lap time with him eg in front of the telly or surfing the net.