I think just go with how you've put it here. It's your Mum's 70th, lockdown means it won't be quite the celebration you had in mind, she's a huge fan and it would mean the world to her if he sent her a birthday message. The worst he can do is not respond. But just imagine your Mum's face if he comes through!Thank you. You've reassured me a bit. Still struggling to phrase it in a way that doesn't sound completely creepy though.