I've heard a lot about the Singapore math series and I'm considering using their workbooks as enrichment for over the summer. I'm not sure what level book to start my son on though. He is in first grade and working at an end of second grade level and is ready for multiplication. He does not know his time tables by heart but he can tell me on his own that 2 minutes equals 120 seconds or that 5x5= 25 etc..but he is not able to actually recite the entire times table. He is also able to do up to three digit addition.

I wouldn't use the workbooks alone for enrichment; if you had to do just one thing, you'd be better off teaching from just the textbook, or doing problem-solving enrichment with the Intensive Practice / Challenging Word Problems supplements. That said, Singapore Math has level-setting tests available to help assess a proper starting level. Good luck.

ETA: Have you checked out the Mathematics Enhancement Programme? It's free.

We love Singapore Math and use it at home. The one criticism I would make is that we find it doesn't explain new concepts very well. I mean, the pictures are good to visualize this is multiplication, etc. But I just told my son how to carry numbers or things like that.
Second grade is where they need to know their math facts for subtraction and addition. You could also consider throwing in a little workbook on Kumon, which is drill and kill. Just a little since it does get boring but it is very good at getting your math facts down.

Singapore has placement tests, and they strongly recommend using them.

For my DD, who is currently in 4th, we're doing some enrichment this summer. I got the 3B and 4A textbooks, and the Level 3 Challenging Word Problems. If she flies through them, we can always order the next set up. (I think she could have passed a good chunk of the 3B material, but she hasn't had significant exposure to weights and measures. Since we're aiming for enrichment, not advancement, that's a good topic to enrich.)

Singapore also has some speed drill / mental math materials that I strongly considered. Mental math is not my DD's strong suit.

Supposedly there's a lot more of this in the Home Instructor's Guide / Teacher's Guide (which we hardly ever crack open ourselves). The full Singapore method is supposed to go from concrete (playing with manipulatives and such) to pictorial to abstract representations / explorations of the same topic.

That could be true. I've only ever bought the one that is white with a blue cover. THat is great but it presupposes that your child already knows what it is teaching.

Can you link to a picture (perhaps finding it on images.google.com or the page where you bought)? There are some books out there that aren't actually Singapore Math but have those words in the title, like this one. I'm not saying that's your book, I'm just curious. I'm also not saying that those books are bad for math practice, not having used them; I just know that they don't contain Singapore Math lessons as found in the textbook or teacher's/home guides.

The white with Blue is what you can find at Barnes and Noble. We used those for 2a and 2b. Ds6 is now working through 3a. We bought the text and workbook online. I really like the textbook. I don't think we really need the workbook, although it is giving us extra practice that I send with him to morning daycare to work on after we go through the textbook selection.

I do recommend using the Singapore placement tests. Ds passed the 1a and 1b tests. We did not give the 2a test to him but started him on 2a (he is in kinder) this past fall. He is now in 3a which still seems quite easy for him. In 2a and 2b, we did not skip anything. In 3a, I feel more comfortable (maybe its because we have the textbook) reviewing in the book and doing a few problems and not all. If its mastered, we continue on.

I've compared the blue and white version of 2a (I think?) that are more widely available to the equivalent workbook from Singapore Math. They cover exactly the same thing in the same order, and the quality of the problems seem about the same. The major difference is that the Singapore Math workbooks showed the concrete part of the instruction. For instance, in teaching the mental math strategies, the workbook showed the process of what should go through your head. In the blue and white, there was nothing. Same thing with introducing new topics -- the blue and white book by and large just popped out with the new problems. The workbook has just enough in there to demonstrate the concept. It's enough that my DS can often figure it out, and if he can't, I can quickly construct something to help demonstrate the concept that I know will be consistent with the expectations on subsequent pages.

Is there a reason not to use the Singapore Math textbook too or instead of the workbook? Is it just that it's not available as widely? The textbooks are really not big, and contain more in the way of lessons. If I had to pick just one, I'd probably pick the textbook over the workbook.