"Bon après-midi !"
"Bon après-midi" and "bonne après-midi" are both correct.
é has an 'acute' accent signifying a short vowel sound, so it sounds a bit like the 'eh' that is attributed to Canadians, whereas the accent on è is 'grave' (said in the French way) and signifies a longer vowel sound. For example the string of letters 'ère' sounds a bit like 'air' in English (but obviously with a French 'r').
A nice way to remember is imagining that you are cycling over a hill. The acute accent looks like the uphill stretch, where you might take lots of fast, short breaths (shorter vowel sound), while the grave accent looks like the downhill stretch, where you can free-wheel and recover by taking longer, deeper breaths (longer vowel sound).
In one lesson "I hope you have a good afternoon" was translated as "Je vous souhaite une bonne apres-midi" but in another lesson ""good afternoon" is "Bon apres-midi" (sorry, my keyboard does not have the accent mark for me to put over apres). Why is it bonne in one case and bon in the other...both describing apres-midi?
They'll know what you mean, but it's wrong. I don't think anyone would make a big fuss out of it, but if you're going out of your way to do it, it may come across as weird.
Also, the sentence you wrote means The dress is the boy. The dress is the boy's could be written like La robe, c'est la sienne (the dress, it's his) or La robe appartient au garçon (the dress belongs to the boy) or La robe du garçon (the boy's dress).
I tested both "bon après-midi" and "bonne après-midi" and they are both pronounced the same. But wasn't bon a nasal sound? How in this situation, it is pronounced like bonne? After over a year of my French learning process, to my ear, bon has been something like (w-oo-l in English, though the shorter version) while keeping it as a nasal ending and bonne to me has be something like (m-o-re). But now seems I have been ... Does "bon" have two variations in pronunciation?
Listened to many variations on many places, like forvo, collinsdictionary, YouTube, Google translate and seems they may vary! Seems French vowels are very regional-dependent...
"Bon" loses its nasal sound when it is followed by a word starting with a vowel sound.
"bon-jour" vs "bo-naprès-midi".
However, "après-midi" can be masculine or feminine, so there is no difference in sound between "bon après-midi" and "bonne après-midi".
The same happens with the possessive adjectives "mon, ton, son".