While it is true villagers have 28 (or some number close) of spots for clothing inventory and that is how you would fully eradicate it (to make sure he never wears it), once you start giving him other clothes, he will likely wear the baby romper less and less.
If you haven't gifted him anything else yet, that could be why he is wearing it so often - because the game only has 2 outfits in his inventory to choose for him.
I often gift my villagers 5-10 outfits each (usually that fit their preferences), and it seems like they rarely wear the gifts they get from each other, and they usually wear what I give them. This is only my experience, so it may not be true for everybody.
One exception is if the piece of clothing has a certain function. For example, Marina gave Poppy a blue track jacket, and now Poppy wears it in situations where she would have previously worn her green track jacket (exercising, etc). I'm not sure if baby rompers are designated as pajamas, but if they are, he may continue wearing it at bed time unless you give him new pajamas. The problem is - if you gift them new pajamas, they will also wear them during the day sometimes and not just at bed time.
I understand why this bothers people, and if it makes you feel better to swap the villager out/reset them, by all mean, feel free to do it! For me, the most helpful thing has been trying to adjust my mindset and making the choice that I'm not going to let it bother me. I often decline deliveries and am careful about what I gift, but I try not to let it ruin my experience if they end up with a bad outfit or something that doesn't fit them.
