As long as you adjust the rest of his food accordingly a slice of toast every so often isn't an issue I don't think. Just don't make it a principle part of his diet.
Our Collie loved the odd slice of toast but he is now gluten-intolerant so can't have it! :(
leelapops is right, adjust the rest of the diet accordingly. i've just read in one of the dog magazines that to a dog one slice of toast is equivalent to 1 cheese burger for a human!!
I heard that white bread makes dogs hysterical - not sure if it's true or not but worth thinking about - I sometimes give my dogs brown crusts with a bit of gravy - a real treat for them.