I agree it probably has to do with the Helsing blood. I wonder if because the Helsing DNA is at war with the Vampire that he maybe becomes a sort of hybrid, allowing him to still go in the sun, eat human food, etc. From a show runner perspective it would make sense to keep him sorta "normal" still.

Most likely it’s being rejected from the van Helsing blood. Left field theory. He’s going to become an energy vampire.

Spoilerino opinion:

I fucking love this storyline so much.

I'm just happy that we finally have some progression on that story line. I also think it has to do with his relation to Van Helsing

He's a late bloomer

Ooo! I like that, I just started the new season a little late, interested to find out!

I think that eventually he'll return to being human. His Van Helsing blood will reject the transformation.

Maybe he’s just immune to being turned.