I would tend to agree with dysgraphia as a working hypothesis. You have a consistent pattern across subtests and time (a year) with regard to written expression: spelling, writing samples, I assume writing fluency, if he had to stop after three minutes to take a break, which made that score technically invalid, but clinically pretty clearly deficient.
Math fluency has dropped, which is also a motor task. This may partly be due to the fact that rising first graders are not expected to know any math facts at all, whereas rising second graders should know addition facts to 18. Consequently, his actual raw score may not be that different, but his normative standing would have fallen, as comparison peers picked up some math skills, while he did not pick up additional motor skills to compensate for the compression of the bell curve in math facts. That is to say, many rising second graders can do basic addition, which is largely what he would be doing on the math fluency subtest, so the ceiling on math skills is pretty low (the test doesn't have enough spread to separate him from his peers on the basis of math skill). This causes the test to become a measure of motor speed, rather than math.
1. Are you in a discrepancy district, or an RTI district? That will make a difference in terms of advocacy. Much easier in a discrepancy district. Another approach in an RTI district would be to argue that there is a failure to progress, as these normative results represent stagnation, or limited progress, in his absolute skills (you'll need to use W scores or raw scores from the WJ for this, which will involve your ed psych again). So his trend line year over year is excessively shallow, which, if it continues, would be projected to result in him eventually falling below normative standards. This would be the basis for instituting interventions for written language, and possibly reading comprehension, even though none of them are currently normative weaknesses.
2. Pretty much all fluency weaknesses have the same intervention: practice, practice, practice. A little every day is much better than a whole lot once a week. Do you have OT recs for increasing hand strength and stability? Get some. (clay, dough, rubber balls, writing with tactile feedback--such as with a finger on sandpaper or in finger paint, or on a chalkboard; writing while propped up on your arms, prostrate on the floor; Handwriting Without Tears, etc.)
Download a free Google or iPhone dictation app and use it to dictate rough drafts or written responses, then (if it's that kind of assignment) go back and edit, revise, or copy by hand. This separates language formulation from fine motor skills.
Accommodations (should you get them) should include supplementary oral assessment, items/problems sufficient to demonstrate mastery only (reduced workload), extended time, and no penalties for misspellings, unless the focus of instruction is spelling.
