I am assuming that your DS has already mastered long division with integers so that it is just a matter of cases where there are decimals in the dividend and divisor. When only the dividend has a decimal, your DS only has to remember that the decimal lines up in the same spot in the quotient so that's also unlikely the problem. He likely only has trouble remembering the algorithm for how many places to shift the decimal to the left for the quotient to correspond to the requisite rightward shift for the divisor. Some kids, like my DD, just memorizes algorithms automatically, so the challenge is to make her go back and assimilate the logical reasoning and develop number sense. Other kids, like my DS, has an incredible number sense, so the challenge is to make him assimilate the algorithms after he can already see where the decimal should go. Perhaps first figure out where your DS' weakness lay. If he has already developed a strong number sense so that he can automatically tell where the decimal should be in the quotient, leverage that to help him remember the algorithm (i.e., by working backwards).
As for the story problem troubles, it is odd that this is only cropping up with 6th grade math unless curriculums really vary that much across the country? Even my oldest, who was in 2nd grade a dozen years ago, started with addition and subtraction word problems by 2nd grade. This approach makes it easier for most kids to master word problems due to repeated exposure over many years and the gradual expansion of the variety (+,-,x,/, algebra, geometry, etc.) and complexity of word problems. Perhaps it would help to focus your DS' attention on just one type of word problems at a time to allow him to assimilate typical language before contrasting the different types. Does your DS have any language processing weaknesses that may impact comprehension of word problems? If he does not, it may simply be an issue of complexity, which requires more cognitive resources including working memory. By 6th grade math, a word problem likely requires multiple operations to reach the final solution. While some kids can manage to execute multiple operations in a single problem when it is laid out for them, having to "set up" the problems simultaneously pushes them beyond their current cognitive resources. I sometimes see this manifest with very young kids who can do calculations in their head but initially struggle when they have to simultaneously write out the solution.
If he has a decent teacher, I would approach her for help since this has the added benefit of a custom fit with his curriculum.
