I think H will feel worse if he figures out he got it wrong and you didn't say something. However, you don't have to point it out as a mistake. I would simply email and say that you wanted to let him know you rec'd the e-card and think it is great and you will be sure to give it to son on his b-day. Then maybe mention something about wanting to have a little party- cake and ice cream or something- mention the day you'd like to do that and see if he wants to attend. Vague enough, not pointing out he made an error, but also giving him the correct info. Just my idea.
