It sounds like you're all set on the 1st grade materials then. As for joining in on Adventures, most families seem to leave it somewhat open-ended, for the younger to join in as he has an interest. Likely he'll have fun doing songs, cooking, and projects. He may enjoy the readings on history, especially if he's done some history already. He probably won't do notebook pages or state sheets all of the time, but there's nothing to say he can't join in if he'd like.
The Adventures materials are very appropriate for grades 2-3, or advanced 1st graders, so there won't be the range of materials you'll see with the 5-year-cycle. The school day is only designed to be 2.5 or 3 hours, including language arts and math.
I think it'll be fine, as long as you remember that his "main" program is solidifying reading and basic math skills.
My one consideration might be Bible. Usually, MFW 1st graders are doing their own Bible, and then joining in on Adventures Bible projects. Yes, a 1st grader can "do" Adventures Bible in many ways, but the depth of understanding would be more perfectly aligned with MFW-1st grade Bible lessons. You'd know your family best, though.
Here are some more conversations about adapting Adventures for a 1st grader:
http://board.mfwbooks.com/viewtopic.php?f=16&t=2082
HTH,
Julie
Julie, married 29 yrs, finding our way without Shane
(
http://www.CaringBridge.org/visit/ShaneHansell)
Reid (21) college student; used MFW 3rd-12th grades (2004-2014)
Alexandra (29) mother; hs from 10th grade (2002+)
Travis (32) engineer; never hs