I don't homeschool, but I am a teacher. One of the things I use to reinforce parts of speech with my students is Madlibs. Also, the Schoolhouse rock grammar DVD is great for parts of speech. You could incorporate other aspects of grammar into everyday writing, reminding your DS to start sentences with capital letters, end them with periods, etc. At 3rd grade, he doesn't need to know more than that. You can get into the more complicated stuff later.
I agree with Peaches above. my oldest is only in 2nd grade right now but we do little more than writing exercises. Some of these I just let her write so that she can experience putting stories down, others we go through and talk about sentence formation, basic nouns and verbs, and punctuation. I do have a copy of "First Language Lessons" which is written for the 1st & 2nd grader. Occassionally I pull out various exercises for us to complete. I think most grammar books start at around a 4th grade level.
On my wish list to get sometime this year are a few Mad Lib books and the SchoolHouse Rock DVDs - dd is just about ready for those.
Mad Libs are GREAT! We also get lots of grammar with our Spelling curriculum, we use "Spelling Workout" by Modern Curriculum Press. For third grade, you would need "Level C". Each weeks lesson includes exercises in such things as synonyms, antonyms, proofreading, creative writing, etc. The student workbook is $14.95 on amazon.com, there is a Teacher's Edition also, but Amazon does not sell it. The student workbook and teacher's edition are available from other homeschool curriculum suppliers. It is a great program! Even my dd likes doing spelling/grammar with this book.
Here are a few great, fun sites we've used alot in the past--some mad libs, grammar games and even worksheets. We've never used a grammar curriculum, just things like this that make learning fun--and lots n lots of reading...lots n lots!
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