Demonstrative adj proceed the noun they modify:
this means 'that boy''
i believe that this is because 'boy' is specified
Such as in the use of a demonstrative adj like: That boy is walking
the concept of the verb is superfluous to the application of the
demonstrative adjective itself
if it had been a substatative adj it would have read 'He is walking'
in which case 'boy' was not specified but 'he' was used instead
i am concerted