It's not only a boy thing. My daughter struggles with this in math, as well -- it's not her favorite subject. She can excel when she is careful, but when she's not... well...
Three things I have found that help. First, when grading her Saxon math tests, she has to evaluate all missed problems. Did she not know the concept (unlikely in Saxon), not read the problem correctly, or miscopy the problem.
Second, we began grading her work together. We sit down together and she gives me her answer for each problem. If she misses one, she must correct it then and there (I can help her if needed, but I usually send her back to the lesson first so she'll learn to do that on her own).
Third, she must write out all steps for her work, as well as check each answer when possible.
This has helped some. Oh, and moving her math to the first subject of the day was good for her as well. I let her work wherever she is comfortable. Some days it's on her bed, sometimes at the dining room table, sometimes on the floor. As long as it's neat and I can tell she is truly working, I'm good with it.
Sometimes it's a challenge to minimize distractions but also keep tabs on how our kids are doing. That's something to strive for.
Hope something in there helps someone!