There is an excellent two-volume 26cm x 24c German series, "Deutsche Schreibschrift" (2002), by Harald Süß, published by Droemer Knaur GmbH & Co. KG, München.
Volume I, titled "Lesen und Schreibens lernen Lehrbuch" is an 80-page manifestly illustrated text and graphics, with three chapters, "Geschte der deutschen Schreibschrift", "Das Lesen", and "Das Schreiben".
My copy of Volume II, titled "Lesen und Schreiben lernen Übungsbuch" has the specifications and illustrations for Offenbacher and Sütterlin scripts. It is a workbook, with a section of font-lined blank pages that can easily be copied for practical learning how to correctly write the fonts of the two scripts.
I ordered and received both books through Amazon, but could only obtain the "Lehrbook" through the German Amazon [Amazon.DE], and not the "Übungsbuch", which I was able to get through the U.S. Amazon (Amazon.COM).
When I received the hard-cover "Lehrbook" the vendor also included with my order a small, thin, Sütterlin-font pocketbook titled "Max und Moritz", a children's book with many line drawings of the two characters adventure(s).
Because the angle at which the letters of the Offenbacher alphabet have to be written, I use an engineering-class plastic 30-60-90 degree drafting triangle to "prep" slant lines on my copies of the workbook's practice worksheets. It takes a while, but I find its use to be quite worthwhile.
Hopefully my experience with resources for lettering and writing in German will be of help to you and to your son. To me, writing with pen and pencil on paper is much, much, simpler and easier than writing with my finger on a tablet which has no lines to guide me.
Oh yeah ... make sure your son "sits up straight" when he writes. No "slouching" allowed, eh?