What I wonder is does your DS spend time translating the word problem into a mathematical equation?
I suspect he's glossing over this step (which DD1 also tends to do) - he gets it's a percentage problem but is knee jerking into mutliplying the percentage seen in the problem by the 'sale price' figure. (in other words not stopping to understand that the figures he's given are clues to the original price - but not direct clues).
I may be wrong - but I suspect some of this problem may be 'rushing' through the work.
If he now gets what to do (based on noblegiraffe's & others advice) - than my advice is the next time he has word problems - get him to slow it right down - write out all steps.
1) what the question is asking (What is the original sale price? Do we know that information - no - we don't know that information)
2) What is the information you've been given - new sale price and what the discount was? We know the new sale price was £63 and the discount was 10% off the original price
3) then consider how to work out original price from that information -
so the unknown original price less (or take away) discount = new sale price. Let's use x for the original price - so the problem can be written as 100% x - (discount of 10%x) = £63.00 which reduced to 90%x = £63.00 to solve it divide both sides by 90% (or 0.90) and you get x = 63 divided by 0.9 = 70
Now one of the issues here is whether your DS can quickly convert percentages to decimals - so he knows 100% = 1/ he knows 10% = 0.1 (in fact useful to also see if he can convert percentages to fractions - so he knows 50% = 1/2 for example). If he is good at this conversion of fractions to percentages to decimals (in any order) - then this type of word problem is much easier.