A bit late to the show, but when I ran this campaign for my investigators the hybrid player was going to take the needle... but Billy had his arm chopped off before he could do the tattoo when another character took it upon himself to deliver justice.
Needless to say, this led to some drama afterwards...
In the definition of Deep One Hybrids, there's a note for a percentage of hybrids to NEVER manifest any of the Deep One characteristics - or as many as you want. In my case, I decided that since his bloodline was so far down the line from the original Deep One, that he wasn't going to change... at least, not for the duration of the campaign. I did give him some advantages, such as a Summon Fish spell, learned in a later adventure, and a bonus to cast it, but you're not under any obligation to have him Change if you don't feel it'd be right for the character.
Again, a bit late, but just pointing out an option if you'd like to use it.
Have fun!
:)