When you have a child who is weak in spelling, you teach him to compensate for that weakness. Recognizing his weakness, he does what is necessary to make it work. Ignoring doesn't work, cramming the words into his brain doesn't work (that's not even learning), nagging about or condemning his weakness, thinking he's just not trying doesn't work. Instead, you teach what needs to be done to improve or make more possible in this area. For him, it's learning ways to compensate for it.
Some people aren't "spellers", at this point in his life, he's just not so I refuse to let him think he can't do it or believe he's not smart...he just has to find a different way. When he doesn't know how spell something, he uses the dictionary. If he has a misspelled word in his writing or other assignments, I show him the misspelled word and he corrects it by using the dictionary. Only after he has tried the dictionary with no success, I will give him the correct spelling, of course, this is case by case basis.