I am amazed at some of us here on the bb. We just let our WASs get away with anything (Sorry, I am just a little pissy today). Not remembering your own children's birthdays is inexcusable. What would be your objective in sparing your H the "hurt" of knowing he messed up on it? I would not of course berate him on it. That would be pressure or control. But you can let him know that you are glad he sent the card and that you will open it for S on his birthday. Just leave it at that. Let him figure out whether he messed up or not. I don't know if DBing really implies that you excuse absolutely every single thing that your spouse does. It does mean to control your reactions I think but to let your WAS know calmly what the facts are.
UD
The 3 laws of DBing:
1. PMA is critical to DBing.
2. Since drop in WAW's PMA leads to drop in LBS's PMA and vice-versa detachment is critical.
3. Validate to raise WAW's PMA and GAL to raise LBS's.