I wouldn't be surprised if John forgot Dean's birthday. In fact, I'll bet he did. I'll bet he forgot Sam's birthday maybe once or twice, but he probably forgot Dean's almost every single year. I know for sure that Sam remembered every time, though. Sam wouldn't forget something like that.<-----except for one year. Sams pissed at their dad for what ever reason and just ignores everyone and relizes it the next day and feels so bad bout it.