Hello and welcome! You are at the beginning of a wonderful journey and you have come to the right place for encouragement and advice!
Unfortunately I can't offer a whole lot of age/grade specific advice as my boys are a little older (just finishing grades 2 and 5). However, this was our first year homeschooling, so I can offer a little insight into getting started.
First of all, what are your biggest concerns as far as whether or not homeschooling will be "doable"? Finances? Family schedules? Caring for younger kids in the mix? Or is it self-doubt? Trust me, no one has it completely figured out and there will be a lot of bumps in the road. If you are leaving the option open to throw in the towel if it doesn't work out in the next few weeks, then you are less likely to work through the challenges and give up too soon.
I was at a homeschool convention last June and heard a speaker say that a lot of people say "I'm going to try it for a year" and then renew that each year. The problem is that when we do that we are not really fully committing to the whole process. Using Sonya's Planning Your CM Education was really great for me in terms of figuring out where I want my kids to end up and how to plan a strategy to help them accomplish just that.
I do know that there are circumstances that make homeschooling only a temporary option for some families. There is a woman at my church who has homeschooled her kindergartener this past year and plans to keep him home for grade 1 primarily due to her husband's work hours. If her son was in school all day, dad wouldn't get to see him all week long. They may decide in the future to send him to ps if dad's hours change or he takes a different job.
IF you really are not sure if you will be homeschooling I would not invest a great deal of time and money on a summer curriculum. I would simply focus on teaching reading, writing and math for the summer and keep lessons very short. Take lots of opportunities to observe nature. Read aloud LOTS of great books! I would not worry about starting history over the summer. Start small, don't over whelm your son, or yourself. Start only one subject at a time and add something new each week until you are doing all of these subjects.
Before anything ellse, pray. Look for God's guidance and trust Him to lead you. There will be lots of times that you will doubt your ability to teach, or think that you must have been crazy to even start, but hang in there. Pray. Trust God. You can do it.