This certainly isn't BTDT advice but when my DS was about 3 and doing more complex puzzles, he would become overwhelmed at the "big picture" and not be able to perform the steps to get there. I wrote out a checklist outlining the steps (i.e. sort pieces into piles . . .) which helped him to focus. We were working on some algebra-esq problems last week and I wrote the steps to solving for the variable. These explicit instructions make him go step-by-step, which forces him to slow down and be more meticulous.
This obviously won't work for spelling tests but may be great to do at home to build good habits. My DS is quite a bit younger than your DS and still very much into rule-following so not sure how well it will transfer.
