"He" is a boy that was working on his sandcastle, not the anthill.
Does it still make sense?
What would be the problem with "him"?. Is not correct at all?
Thanks again! I don't mean to be "heavy" but I really need this info!!!
Mesil you see the position of ‘he’ in "He is a (the) boy that was working on his sand castle, not the anthill." makes it the subject of the principal clause. The pronoun must therefore be in the nominative case (form) ‘he’ and not the accusative case ‘him’.