I have a kid with red hair too and everyone asks, "Does red hair run in your family?" The question is made easier by the fact that my wife and I do have relatives with red hair and you can see red tints in both our hair. I can imagine how frustrating the questions would be without having an easy answer…
The quick answer is that it is very possible (obviously!) to get a redhead from blonde and black haired parents. I'll go into the details below but next time you get a question about this, maybe you could answer:
"This is actually pretty common. Because my husband and I are both freckled, we probably are carriers for red hair. We had a 1 in 4 chance of having a redhead and we couldn't be happier to have beaten the odds!"
Hair color in general is really complicated and poorly understood with the exception of red hair. Many cases of red hair can happen by the simple dominant-recessive model I'll discuss below.
Before getting into the specific genetics involved, let's first talk about hair color in general. Hair color is determined by the amount of two pigments called eumelanin and phomelanin that are in your hair. The amount of eumelanin in your hair gives you a range from blonde to black—a little eumelanin and you are blonde, an intermediate amount, brown, and a lot, black.
Red comes into the equation with phomelanin. The more phomelanin in your hair, the redder it is.
OK, then, hair color is a mixture of how much eumelanin and phomelanin is in your hair. For example, strawberry blonde is a little of each, auburn is lots of eumelanin and a little phomelanin and a redhead is very little eumelanin and lots of phomelanin.
How does the body decide how much of these melanins to put in your hair? Genes, of course. (If you are interested in some current theories on inheritance of hair color besides red, please see the link below.)
Humans usually end up with very little phomelanin because of the product of a gene called MC1R. What MC1R does is cause phomelanin to be changed into eumelanin making red hair pretty rare.
When someone has both of their MC1R genes mutated, this conversion doesn't happen anymore and you get a buildup of phomelanin, which results in red hair (as well as fair skin and freckles).
So, your son may have two mutant MC1R genes, one from you and one from your husband. The fact that you and your husband have freckles may strengthen this idea as I have read that people with a single mutant MC1R gene don't necessarily show red hair but often are freckled.
So it may be a simple case of dominant and recessive traits. You and your husband each have a working and a mutant version of the MC1R genes making you freckled but not giving you red hair. Your second son got two mutant MC1R genes while your first inherited either one or no mutant copies. By this model, you had a 1 in 4 chance of having a redhead.
As to why you haven't seen it in your family before, it depends when it entered your family and simple statistics. For example, let's say your family has a single copy of the mutant MC1R gene; no one has two copies of the mutant gene. If everyone then marries people with two working copies of MC1R, then any children produced will have either one or no copies of the mutant gene.
Even if they did marry someone with a single mutant copy, there is still only a 1 in 4 chance of producing a redhead. It is important to realize that if you have four kids, this doesn't mean that one will be a redhead for sure. It doesn't matter what came before, each child has a 25% chance of being a red head.
You can see how even if there are no known redheads in your family, you can still be a "carrier" for red hair. Hopefully this answer can help arm you against the inevitable questions.