I think that still may be spoilers, he doesn't flat out know that Meliodas is an actual demon yet (I may be wrong).
2019-08-23 23:37:26 +0000 UTC
This isn't really spoilers since it doesn't change the plot but it answers your questions Meliodas is fully demon Dreyfus is different he is not a full demon. I don't know what would happen if someone drank Meliodas blood maybe manga readers know.