Bearhug
To me that sounds much more like your husband is "to blame." When you are the minority language parent (both in terms of input, as dads typically are, and in terms of community support, as non-english speakers in England typically are) you really do have to make a stand for your language. So, when your son doesn't say "dobrou noc", your husband has a choice - either to let it go (imo a bad idea unless it is because he is picking his battles) or show his son that, for him and for his relationship with his son, speaking Czech is valuable and important.
As I've said before, I would do this either by just 2pretending" my child had spoken French to me and responding in kind ("night night daddy", "bonne nuit cherie, fais de beaux reves" etc) or (now she is older, pretending not to understand ("night night daddy", "pourquoi tu me parles en anglais? je te comprends pas", "bonne nuit papa") - very light and non-challenging in practice, especially with a smile and at times when things are happy anyway, but very effective.
It is great that you support the Czech but the message that may be coming across is 1) mummy is defending Czech because daddy can't be bothered; 2) mummy claims Czech is important but doesn't use it herself - no one practises what they preach round here!
This is not intended as criticism, more as a warning - in families I have seen where the second language does not "take" as well as it could, it is nearly always because the parent representing that language does not show the child that, for him/her, speaking that langugae is a big deal.
Of course, when the language does take hold, it can actually improve the relationship as there is then a certain complicity between parent and child, thus creating a virtuous circle.