Ah. Perhaps he had a fright when he was little. Perhaps psychology is needed - if he needs to go outside to the toilet, could you not go with him, show him there is nothing to be afraid of?
i do go out with him at night, i have an outside light,he is an anxious dog, ummm geting another dig in, im asking a querstion about a pet, what has that to do with ones profession ? thanks for helpful answers.
You need simple positive association with darkness- so what I'd suggest is go out into the garden when it's merely dusk and he's not anxious- play with him, throw sticks, balls, whatever until it gets dark and as woofgang says feed treats and maybe take him for a walk then. Hopefully if he's already enjoying himself he'll feel better about it getting dark and won't notice as much as if you try to sling him outside when it's pitch black already. You'll have to persevere with this for a while and I know it's time consuming but I think it will work, at least that's what I'd do with my dog in similar circumstances.Good luck.