I also have been telling DS he really needs to show what he knows to encourage the school to give him challenges. I don't like saying this. It feels like pressure and the wrong motivation. But it is the reality.
We have this exact same issue with DD8 and it's been going on for a year now. She used to be a staunch perfectionist and we worked really hard with her on that. Last year in 2nd she started making simple mistakes and rushing through work "in order to get onto something more interesting". All the while she was asking for more challenging work from her teacher. The teacher told her "Do the easy work well and you'll get more challenging work.". DD complied and got the necessary 100% on math tests given per unit to go to math enrichment. This worked for awhile, then by the end of the year she was back to where she was but her attitude spread to other areas. She said "Enrichment isn't challenging so what's the point of doing well on the tests to get to go?". We had a hard time arguing that logic.
This year she's basically given up on her school being able to challenge her. She continues to make simple errors due to rushing but the school believes it's because she hasn't memorized her math facts (she uses mental math tricks). She doesn't do her best work and continues to rush because she just doesn't see the point. Part of the issue is that we aren't output oriented parents and are really reluctant to force her to play the game. We feel really stuck.
DD has decided she wants to school at home next year so she can "learn at her own pace".
I'll be watching this thread closely!