In certain situations I find my mind locks up, sort of like when a computer freezes up. One suggestion I have would be for your son to try doing things in smaller pieces, like say 5 or 10 minutes at a time. For me, sometimes looking at things as a whole, or for longer time periods seems very daunting and can cause brain freeze.
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?No great art has ever been made without the artist having known danger? ~ Rainer Maria Rilke