As far as he knows and we know, he is still a vampire spawn. The tadpole is arresting everything about it except for the thirst for blood. As far as the monster manual CR and hp, just... ignore that... Larian doesn't seem to be using the standard CR and hp for things.
He either is undead or he is not undead. And if the tadpole changed him to aberration then it should have changed everyone in the party. Being undead means that you are a corpse. You don't eat, breathe, or sleep, you are immune to certain effects like poison and necrotic energy, you are not affected by healing magic, and clerics can attempt to turn you. You could argue that the tadpole removed his sunlight sensitivity to make Astarion stronger, but it actually seems to me like he got nerfed hard.
