That's great news, Jo, and great insurance in case your son needs a transplant. Since he got half his chromosomes from each parent that means that you and your husband have HLA types (genes) in common. It's strictly determined by inheritance over the generations.
Parent/child matches are fairly uncommon but it's much more likely when parents are from the same ethnic background and have ancestors from a population that tends toward more homogeneity and less genetic diversity, for example if your families have both been in Australia for many generations. Matches are also more likely if your genes and your husband's genes are more common in the gene pool. A single rare gene makes a match much less likely.
In you're interested in more detail here's an explanation that might help. Six-of-six matching is based on antigens named A, B, and DR, which comes in sets. For example, using made-up numbers, you and your husband might have these HLA combinations:
A1 B2 DR3 -- you got from your mother
A4 B5 DR6 -- you got from your father
A7 B8 DR9 -- your husband got from his mother
A1 B2 DR3 -- your husband got from his father <-- by chance matches one set you have
The matching sets of you and your husband aren't enough. Your son would have only a 1/4 chance of inheriting that combination. It was equally likely that he'd get
A1 B2 DR3 from you and A7 B8 DR9 from your husband
A1 B2 DR3 from you and A1 B2 DR3 from your husband
A4 B5 DR6 from you and A7 B8 DR9 from your husband
A4 B5 DR6 from you and A1 B2 DR3 from your husband <--- match with you
So you were lucky to share typing with your husband and lucky again that your son inherited the right combination. The testing center might be willing to share your HLA typing results so can see the real data. They did that for us when my family members were typed, although in our case we didn't have a match.
No matter which factors led to the match it's a bit like winning the lottery - a long-shot chance over which you didn't have control but for which you're very happy to use the winnings!