I want a word for a person who doesn't know much but is extremely good with words. The words he uses are pompous but he doesn't write anything concrete. If you are wondering, it's not fiction. I am talking about the people who publish their articles in newspapers but do not say anything substantial. I need a colloquial word, and it should be derogatory, not a compliment about the person's vocabulary prowess.
Talk that is pompous but not concrete is vacuous. ( It is pumped up but not solid, so it is empty—or full of air like a balloon.)
If he uses magnificent words, he may be grandiloquent http://www.thesaurus.com/browse/grandiloquent —and still vacuous.
If he makes unsubstantiated, fallacious arguments that only appear to have any substance, they are specious. http://i.word.com/idictionary/specious
If he makes big, vague promises without delivering any workable plans, he might be said to be
- all sizzle and no steak
A sophist. Someone who is good at rhetoric and persuasion, even though fallacious. More concerned with the art of speaking than truth and content.
grandiloquence - a type of talk that is pompous and bombastic, full of pretty sounding words and elegant turns of phrase that add up to nothing