I am gathering curriculum for homeschooling DS7 next year. He is reading in middle school level. He is also a very good speller. I am going to do vocabulary with him through the Michael Thompson series. Is it necessary to get a spelling workbook for him? Do you have any recommendation?

Since he sounds like a natural speller, I like the approach in A Well Trained Mind with some dictation and a running word list in the front of a notebook to copy/review words he habitually misspells.

If he needs something more, this All About Spelling program looked good but expensive. There are sample lessons to look at here:
http://www.all-about-spelling.com/spelling-lessons-samples.html

There are also some good articles on that site including this one you might like called Homeschool Spelling.
http://www.all-about-spelling.com/homeschool-spelling.html

I'm not a homeschooler yet but I practice just in case.

My son's school uses Words Their Way. I don't know if it's practical to use when homeschooling. They take a pretest to determine what they don't already know, and start from there. It teaches patterns in words rather than a random word list.

Another idea is to go over the spelling rules and exceptions. It can be neat for a natural speller to learn the "whys" and be conscious of the patterns.
http://www.dyslexia.org/spelling_rules.shtml

Thanks! I think maybe the spelling rules and exceptions will work for now. I have heard from teachers and parents that kids learn the spelling of words and forget them in a month. I am sure spelling tests work well for some people. However, I am thinking spelling words have to be meaningful to ds in order to stick.