Perhaps you can ring (or pop-in if close) to the sonographer's and say you've changed your mind and would really like to know. You can still let it be a suprise for DH at the birth, but if he really feels strongly about it - I can understand why it would be hurtful for people other than you to know about the sex of his baby before him if that's a bit deal to him, would you consider keeping it from your mum and BF and just satisfying your own curiosity.

My DH doesn't come to the majority of my antenatal appointments due to work etc - so the ladies' suggestion of talking to the midwives about your pre-determined pain relief options is a good idea - they can make a special note so that when you come into the hospital whoever delivers will know. The MW at your last birth probably assumed you had strong ideas about not having one prior and had asked DH to stop you IYKWIM - so if they know this isn't the case, you should be able to avoid this again.

Hope whatever you decide you feel better about it all soon. All the best.