How do you deal with a student who has good writing skills but spells poorly?

Agreeing with all other suggestions made here, I would like to add that students are prone to make errors in spelling of some typical words which are either long or have peculiar spellings (mostly words borrowed from other languages into English). it is advised to teach some common spelling rules, like, word-formation rules, pluralization, category-changing (noun to verb, noun to adjective, adjective to noun, etc) and provide your students with a list of common borrowed words, like French words, Latin words (index-indices, thesis-theses; analysis-analyses; chauffeur, etc.) I suppose that will help.       

by making him type what he has written on an oxford English dictionary word file.This would make him get the hang of his errors & give him urge to do the correction 

He needs more and more fun Reading

He needs more training .............

Give him extra spelling exercises like cross words' puzzels.