Welcome to the forum, greenthumbs.

You've received great insight from aeh.
I'll just add a link to a paper which may be of interest:
https://www.future-ed.org/paper-vs-online-testing-whats-the-impact-on-test-scores/FutureEd
May 29, 2018
The 39-page report linked as "
somewhat surprising findings" in the article above may also be of interest:
https://caldercenter.org/sites/default/files/WP%20190.pdf?platform=hootsuiteIs the Pen Mightier Than the Keyboard? The Effect of Online Testing on Measured Student Achievementby Ben Backes and James Cowan
American Institutes for Research/CALDER
NATIONAL
CENTER for
ANALYSIS of
LONGITUDINAL
DATA in
EDUCATION
RESEARCH
Working paper 190
April 2018
The above paper and report do not discuss test taking strategies for the general population or twice-exceptional students.
However they do compare the test taking experience in paper and digital forms.
They also share that the limited data suggests that gaining experience and familiarity with the online test format tends to diminish any gap in scores (2nd year results as compared with 1st year results).
You and your son may already be familiar with general test-taking strategies recommended for students with ADHD (although not related to digital tests). Here is a link to that type of information, which may be of interest, although it may not be new to you. Some of the ideas may not apply to your son, for example: the accommodation for extra time, the use of a timer. Some of the ideas apply only to paper test experiences, for example: writing neatly, going back to questions. Other ideas, such as visualizing the experience, and calculating or estimating the amount of time per question may be skills not only for test taking but also transferrable to other areas of life.
https://www.additudemag.com/test-taking-strategies-adhd-student/Test-Taking Strategies for Every Exam TypeBy Michael Sandler
Updated on May 31, 2021
Students with ADHD tend to crumble under the pressure and information overload of bigs exams. Use these specific tips to prepare wisely...