Whether or not I believe in it, there is such a thing as a summer learning slump -- plenty of studies have shown that kids, generally speaking, do lose some of what they learn and have to catch up when fall comes. And catching up can be stressful for kids. So why wouldn't anyone encourage some practice over the summer to reduce their kids' need to play catch-up in the fall?
This is from Cornell University (so the source is NOT some company trying to sell workbooks!):
"A systematic review of 39 studies published in 1996 found summer loss equaled about one month of classroom learning, and students tended to regress more in math skills compared to reading skills....Since then, additional studies and reviews have found similar results. A 2007 study by researchers at Johns Hopkins University examined data from a nationally-representative sample. They found the achievement gap at ninth grade mainly traces to differences in summer learning during the elementary school years. And a 2004 study found that all achievement gaps among students tend to be exacerbated by summer breaks."
So it doesn't matter what we think about it, there is plenty of data that there is learning loss, and certainly by high school that loss really shows up when kids are back at school in the fall.
I know your son is still very young, so of course his summers should be at the playground and the beach! I also am sure you'll keep reading to him and pointing out words and going to library story times and museums and more. Those things go on all year long for younger kids like yours.
As a parent of a middle schooler, I have to say that as kids get into middle elementary years (around 3rd grade), things change.
School itself is just not like what we experienced as kids, and parents of younger children haven't found that out quite yet, I think, based on a lot of what I read on MP (not just your post here, but many posts over years). Kindergarten? Much higher expectations of what they should know by the end of K than when we were in K! And that means there's more for them to forget between K and 1, and 1 and 2 and so on...Where the need for some summer "maintenance" academics really seems to kick in is between 2nd and third, or 3rd and 4th, depending on the kid, the curriculum and the school -- and the family's own philosophy.
We sent our daughter to a math tutoring place (very mellow, rewards given, we set our own schedule there, she went around twice a week) starting the summer after second grade so she could get a bit ahead on learning times tables and basic multiplication, which helped her be a little ahead at the start of third grade, and made the transition to multiplication easier for her. She's gotten this tutoring each summer since then, because as my husband says, "Math is a muscle and has to be exercised or it gets weak." She likes it, and actually asks to go, and now is old enough she really sees the benefit of just practicing math through the summer months. She also happens to love to write so she does a summer writing program for a week that she loves (third year). But she also does Girl Scout camp and dance lessons and sits in the back yard for hours reading. So it's not all academics, all the time. She still gets to "be a kid" but being a kid does not mean doing zero math for three months.
I too remember summers of being home, reading a lot, watching some TV, and roaming the neighborhood and going on family trips. But times are different and certainly in our area, schools are demanding and the expectations for kids to move faster into math and reading and comprehension are not at all what the expectations were when we were kids. Keeping up basics during the summer actually can reduce the child's stress at the start of school in the fall. I know that it reduces my kid's stress over math, and I have seen the same happen with a number of other kids we know -- a little academics over the summer, usually in fun settings or casually at home with some rewards involved, can help cut down stress later.