Here's my read:
First, remember, you don't have to take your AoO just because it's available.

If you choose to take an attack while under Sanctuary, once the attack action is completed, Sanctuary would drop ("If the warded creature makes an attack, casts a spell that affects an enemy, or deals damage to another creature, this spell ends.") It would drop even if you had a way to do a spell attack for damage, not just a melee attack.

But, the opponent moving 5'+ would take the booming damage as well if they willingly moved. So, I'd say the net effect is - you could do your AoO or not, Sanctuary should still drop either from AoO damage or from your Booming Blade spell doing damage to him when he moves, I would thing.

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