I would explore Kumon, I know many people who absolutely adore the Kumon pre-writing and early writing. I have never used them myself, but I have heard the praises of
these books sung by many people. Since Kumon is common in your area, then I would investigate this option and their competitors.
I'm not saying to enroll him in Kumon, but I would look into some of the materials available in my area.
I think I would take a 2-pronged approach to my sons math education at this point.
1: Mathematics:Can your son skip count? Does he know 1-1 correspondence, does he know what it means to add, subtract, multiply and divide? Can he read numerals? I would begin to work on those issues and I would probably begin to use Marshmallow Math with my son. If he is counting and recognizing quantities, then it is a good idea to begin teaching him arithmetic and I would use the outline provided in
MM to get started. Teach him to add and subtract by 1, 2, 3 etc all the way to ten. The soroban is a great tool and many parents are exposing their kids to it. But many parents don't get far because they can't use it fluently. If you can use the abacus well, then I would lean on that.
2: HandwritingI would begin working on his handwriting ability. You can print many pages offline for free or you can buy a book or you can do both. I would simply make it a part of the day. I would
require that he spend 5 minutes a day tracing/coloring/writing with my direct guidance. If he is resistant than be gentle, set a timer and give him praise when he is done. Ultimately, I am having a hard time visualizing a child being really successful in a workbook based curriculum if writing isn't fluent and easy for them, but I do NOT know your child and I am NOT "The Authority" on these things.
These are just my thoughts.