It may be that he will need to make some adjustments to the Singapore method, too. I started my #1 on SM with book 2B (it sounds like your son is on 3A or 3B), and noticed that the first year was markedly harder than the year my #2 (who started from kindergarten with SM) did the same levels. There are terms and strategies that he may not be familiar with, that he would have encountered if he had gone through the earlier levels. And patterns that he already has, which SM may require him to re-learn or adapt. In our case, we did not see any need to go back to catch those, it just made our initial foray into SM a little more uphill than it would have been otherwise.
I would echo the general rule of thumb about homework, which is 10 minutes for every year of grade, all subjects combined. In addition, I would point out that homework at the elementary level is valuable solely as a means of building work habits. The research does not support any academic benefit from doing homework, if classroom instruction is effective. (In fact, there is some negative correlation between amount of time spent on homework and academic performance, past a certain point. At the secondary level, the volume of reading and content to be covered does support assigning some homework, but no more than 2 hours.)
Some possible homework adjustments: complete only sufficient problems to demonstrate mastery; hard time limit of 30 minutes on homework, with parent signature to indicate that this is what was completed as of the time limit; first x items written by child, remainder scribed responses; odd/even items only; circled (priority) items only.
I also notice that the SM teacher's guide lists most days having 2-5 pages of homework from the workbook, extra practice, or CWP books, plus 1-3 pages of in-class practice from the textbook. It sounds like the teacher may either be assigning in-class practice items as homework (not advisable, as that removes opportunities for guided practice and discussion), or possibly doing two lessons a day.
