This gets into the whole philosophy of math education, but I have pretty strong feelings about not pushing the "rote memorization of math facts." Why? Well two main reasons.
First, ending up in a situation like your ds's, where this becomes a stressful, frustrating and/or endlessly tedious prerequisite for advancing to more interesting math, can taint a child's entire experience of math. The child begins to think of himself as "bad at math," and math as something difficult and hateful, when in fact arithmetic facts are the smallest part of math, bearing very little relationship to actual math aptitude.
Secondly, I think that there's a bonus that comes of not having the "facts" easily retrievable via rote memory while in the early years of math education, in that a child thus gets ongoing practice at deriving the solutions through mental math exercise. If "nine plus six" doesn't trigger the instantaneous delivery of a memorized response, your child will have to think through a process like "nine is one less than ten, so I can make ten by taking one from the six, so then I have five left to add, and ten and five is easy, that's fifteen, because fifteen means one ten and one five." He's just used the concepts of place value and regrouping. It might have taken him four or five seconds, but that's four or five seconds of exercise of his mathematical muscles. Done thousands of times over, that type of efficient use of mental math tricks and concepts will pay dividends later when he's trying to factor algebraic equations or convert something into scientific notation.
We are pretty much unschoolers, though my kids did enjoy using math curricula. All four of my kids were conceptually way beyond the 2nd or 3rd grade level in math before they got the arithmetic facts to an instant recall level. For fun they sometimes tested themselves for speed and accuracy, but we never did anything like drill. Because this made them a bit slower, the work they did with more advanced arithmetic was not high-volume repetitive work -- that would have been really hard for them. We used Singapore Math which was low on repetition, and I figured if they could show me perfect accuracy on two or three challenging problems they had the concept down and they didn't need to also do a dozen less complex practice problems. Gradually the rote recall just gelled, through naturally using the facts over and over again. In a school situation is might have been a problem, because they would have had to do pages and pages of practice, and wouldn't have been allowed to use a printed multiplication table to look up the facts when necessary. In our home, though, without large-group testing pressure, without endless pages of assigned practice, they were able to learn more advanced skills and many interesting things long before the rote recall was in place.
As a result my kids all really enjoy math, and I think that's the biggest indicator of success. We say this all the time with literacy ("what's important is to develop a love of reading") but I think it's just as true with math. Math is very cool! The inter-relationships and patterns are brilliant, the way it lets you solve problems in three different ways and end up with the same solution, that's exciting! You can build increasingly abstract layers atop each other by adhering to logical principles, and end up with solutions to incredibly complex problems, solutions that can't be intuited but are known to be correct: so amazing. There's no reason the world of joyful math discovery needs to be held hostage by "weak" rote memory skills.
At a 3rd grade level my kids were probably similar to your ds. By the time my were at a 5th or 6th grade level they were pretty fast at working out any necessary 'facts,' such that they could cope with school-like exercises without being unduly slowed down. By the time they were at a high school level they either had the facts memorized or were so fast at deriving them mentally that it amounted to the same thing. But the kicker is that they enjoyed math all the way through, advanced very quickly with their conceptual skills and have excelled (or are excelling) in accelerated advanced-stream math through to the senior level after entering the school system at a high school level.
All of which is to say that in your situation, having tried a few different strategies and seeing that rote memorization is not coming easily at this point, I would let it go for a while and move ahead conceptually, re-igniting some enjoyment of math.
Opinions may vary.
ETA: Oh, and lest it seem from what I wrote above that I was actively discouraging memorization with my kids, that isn't the case. I just encouraged what memorization they seemed ready and motivated to do, which in our case was a lot less than they would have been pushed to do by a traditional math curriculum. We had a table of the facts, laminated, and when one fact had become thoroughly memorized, we celebrated it by covering the answer up with a sticker. As the stickers gradually accrued, my kids would occasionally identify a particular fact or two that wasn't yet learned that they wanted to be able to remember, and I would help them focus on committing those to memory. "This week, I want to memorize 7x8 to fill in the gap on the multiplication table that's annoying me " was a far more effective strategy for my kids that going through a stack of flash cards.
Miranda