My preference for a partner is that he doesn't have social media. I don't (except for reddit and linkedin), and believe it's the most healthy option.

I also don't have on whatsapp the option to let others see the last time I was online (and therefore I don't see that info about my contacts either). I don't have anything to hide. I just don't need it and feel better without it.

Now, I did have a partner before who had no social media, and this was at a time when I still had Instagram. I was fine with him having it or not. I knew him for 7 years when I entered a relationship with him and was with him for ~7months when I said to my very insistent sister that he did not have social media. My sister looks him up... he did have a private IG. I was devastated, he had to be hidding something to lie about that.

I reached out to someone that I knew had his IG (we both worked with this person, but no one at work knew about us). Very unconfortably I explained the situation to this person, and asked if she wouldn't mind to let me see the profile. She understood and let me. What I find there, is that he has a child. Again, I was devastated. I did I not know? Was he still with the child mother? And the obvious intention to keep this from me. The rest of the story doesn't really help to the reason why I explained it up to this point, so the point is:

Is it possible that you're uncomfortable because you don't know if he really doesn't have social media?

Not having social media does not mean someone is hiding something, at all. I would be more worried if the person tried to keep the relationship a secret, or keep you away from stuff that's part of his life (friends, family, activities, etc).

But hey, it is an unusual choice, so if I was feeling like you are, I would casually ask "hey, why did you choose not to be on social media? What are your views on it?". I think it's an interesting subject to discuss (with whomever), and maybe it helps you understand his reasoning behind the choice.