We switched from Singapore to Saxon and I cannot say enough good things about Saxon. I just could not find a good way to teach Singapore in a way my 8 year old would understand. Now, my daughter is technically in 2nd grade, but I bought the Level 3 and it's a great fit. If you're not sure, there are placement exams to find out where your child should start. I love that the entire lesson is scripted. It's also VERY repetitious, but that is good for my daughter to make sure she remembers the basics even while we're learning new skills. Now we kind of modify it (in part because I have 2 other children to teach, and also because it is a VERY long math lesson), we don't always do EVERYTHING every day, and sometimes we only do 2 of the 3 worksheets they recommend(she has mastered basic addition and subtraction so we only do that occasionally for review rather than every day). So you can still modify it for your student's attention span so it's not quite so much work in one day. We love it!
Saxon does a great job of teaching children math. The math principles are taught in a clear, easy to understand manner. Plenty of review is given to strengthen skills. I have confidence that my children will do very well in math from using Saxon.