Interesting question.
Interesting response...although I'm not convinced that the various meanings can be considered as 'antonyms'. There are many, many words that have multiple meanings, and not just in the English language. In this case, 'frightful' as the definition of 'terrific' is not really an antonym to 'extraordinary', or 'magnificent' (Merriam-Webster). The root idea may be something along the lines of 'excitement', that is, the arousal of the emotions, from fear on one end of the spectrum to joy on the other. 'Very good' in no.1, and 'very bad' in no.4 are judgmental terms--are those quotations from your dictionary source?
Ultimately, what is not 'stupid' is the notion that the meaning of a word (or a phrase, or a sentence) is determined by its context.
Also, 'terrific' would not be the best word to use with your stepson if you're trying to be ambiguous, because no matter what definition is inferred, there is a suggestion of intense excitement.