Dawn,
I'm sure others will chime in! Math has always been a tough subject for my son (now 9). I was told not to switch math curriculums if at all possible since it makes the process of learning math harder for the kids. Obviously, when something truly isn't working, it is a good idea to switch. My son also struggles with the fast pace of Singapore, but I've found by being patient that by the time the concept is reintroduced in the next book, he is able to understand it better. Also, he is only in book 2b as a 9 year old. I have learned not to stress out about this. He does not have any developmental challenges, math is just tricky for him. I've read that math sometimes does not start clicking until kids are 10 anyways.
If your younger son liked Singapore, I would see if you can make that work again. I like Singapore Math's method and design, and that it does not overwhelm the child with tons of problems to solve. What we do to make Singapore work is to take it at the pace that keeps the child below the "incredibly frustrated" line. This varies from day to to day, concept to concept. I just keep a sticky tab in the guide and work at the best pace for my child. Sometimes, we get through Carrie's scheduled pages for one day, sometimes we don't. When a particular math topic is really tough, I let him work on math games online or on the ipad as a reward and to give him extra practice in a way that is not frustrating. I try to find games that deal with the topic so that he can "play" and learn at the same time. We might do this instead of scheduled math for a day or two. Especially simple math games that help him practice adding and subtracting. (He HATES subtracting!

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We have many conversations about perseverance through our math times. But I don't want him to hate math, so I try to balance pushing him with keeping him encouraged with what he can accomplish. Anyways, at least for your younger son, if he has that much Singapore under his belt already, it might be an easier transition for him.