I have an FAQ that offers advice for dealing with reluctant husbands here: http://www.carolinah....com/afaq6.html
Talk to your husband to see if he'd agree to a year as a trial period. It takes a while for most families to find what works for them (structure vs less structure, textbooks vs hands-on materials, etc.). If you start out one way and it doesn't work, you won't have much time to experiment and you might think homeschooling isn't working for you, when the reality is that you just haven't found the method that works best for you yet. Trying to figure it all out in a couple of months (and being under pressure to make it work in such a short time-frame.... or else!) may not be a good start to homeschooling.
All that said - if summer is all you have to work with, then make the most of it. Go to museums, the zoo, historical parks, etc., and be sure to take pictures, collect brochures, etc., to document all the learning that takes place. Keep a list of books that your son reads, note any documentaries or educational television he watches, any discussions about the Great Depression, Vietnam War, etc., that he may have with people who've lived through the period, etc. I don't know if I'd use any textbooks or school-ish type of resources at this point because 1) I think it's important for your son to see that learning happens all the time, and 2) he'll get hit with that when/if he returns back to school this fall and may resent the fact that he didn't get a summer break.
Also, I encourage you to join us for any events we do during the summer. Many, many reluctant husbands and grandparents have been turned around after they see the results of homeschooling by those who've been at it a while. I've been homeschooling for over 20 years, with children who've never been to public school, so my older kids (ages 27, 21, 17) are used to talking to people who are unsure about the whole thing. I can't tell you how many times uncertain husbands, wives, or grandparents have told me that talking to my children did more to convince them than anything they've read or heard up to that point.
To answer your questions:
You're welcome to join Carolina Homeschooler for the 2014-2015 year, even if you end up enrolling in public school in the fall. You won't be able to get a refund, but you'll be able to attend our activities and online book groups, etc., this summer.
Definitely keep track of learning that goes on during the summer. Remember to count everything, even the unexpected learning that happens. I remember one lazy Sunday we turned on Discovery Channel or something and a show about NASA was on. It sucked us in and we ended up watching it all day long (it turned out to be a "history of the space program" marathon). It was totally unplanned, but we all learned so much from it (see recomendations above about books, museums, etc.). We don't have an hourly requirement as far as counting days goes.
I hope this helps! Let me know if you have any other questions.