I assume that this could also be translated as "my only child". A person may have only one son and many daughters.
Unique means that he is unlike any other. He doesn't have to be the only one but you may consider him exceptional for some reason. Of course, we know that every individual is unique but it is a way of referring to some exceptional quality.
however, there is no such distinction in Portuguese, both "unique" and "(the) only (one)" translate to "único/a".
I gave that answer and it was marked wrong. It shouldn't have been because it's technically correct as a translation. Nothing wrong with it...
I made the same mistake.. but that's because I'm french. Reading Coayuco's comment I think he's right, "unique" in english means more than "without brothers and sisters" and "only son" is the proper translation. It's a bit of a pain to be marked wrong here because of a fault of english (while we had the portuguese right), but it's the name of game :).
The meaning of some adjectives changes depending on whether they come before or after the noun.
Because it is referring to numbers (or an amount/value). Similar to the days of the week, segunda-feira, quarta-feira, cinco-feira... Mais, muito, menos too. Proxima also usually precedes the noun. Um euro = 1€
It happens when we use possessive adjectives. I'll find the link where there is more explanation about it ;)
Paulenrique answered well. I just thought it might be helpful to point out that while the adjectives usually come after the noun, an adjective can be put before the noun if it is the thing you want to stess. (As in if the adjective is the whole reason you were talking about the noun in the first place)
That's not how we would normally say it in English. If you mean a son that is unmarried, you would use the word 'solteiro'