DS needs to learn to show his work- the problems are getting harder, and doing it in his head may not cut it for much longer.
Does he make a lot of mistakes doing the problems mentally?
Be careful, because all this "show your work" may not be working with gifted kids, because problems are way too easy for them. In my experience, they feel the need to write down their calculations only when they absolutely have to, when the problem is interesting but hard to figure out mentally.
As for the SM... I don't wish to disappoint you, but it will probably get worse. In my area only private schools implement the so-called SM - namely, the program, which is called "Math in Focus - Singapore approach". This is just a weaker and diluted version of a real SM, but approach must be the same - a lot of meaningless homework, a lot of writing, drawing, and a little of understanding. The main idea of this program (at least, in our local implementation) is "do not think, just do 10 pages of homework exactly the same way your teacher told you in class"
A couple of my students were struggling tremendously last year with this program mainly because of a huge amount of homework. 4th grader, for example, had 10(!) pages of homework (which requires a lot of drawing) every evening, and by the end of it he was like a zombie, unable to think and unable to explain what his homework was about.
To make a long story short, I wouldn't recommend another skip namely because of a huge workload demanding a lot of writing and drawing. However, my advice is based solely on my experience with local schools, in your area the program (or rather its implementation) may differ.
