i'm sure his intentions weren't to hurt your feelings, it is his mother after all and it may have just been hard for him to bundle up such a secret inside of him. However, he must have told her not to tell anyone (which shows he was indeed thinking of you and the hopes of keeping it a secret until you are ready to tell) because his Mother acted like she didn't know a thing! She can keep a secret for sure. It is a lucky thing that the person he chose to confide his secret with is very trustworthy because she could very well have told everyone she knows!
Don't be too upset with your DH

afterall, you have said yourself that you 'spilled the beans' on your pregnancy to the forums on the internet. How hard would it be for you to keep such a secret if you didn't have us on the internet to confide with? I'm sure the secret was too much to keep inside of him that he had to tell SOMEONE!