Thanks for the thoughts so far! The Baby Bjorn seat that goes on the toilet has awesome reviews but it is hard plastic so it's good to know Oliver doesn't care for it.
I might get a cheaper one that is cushy b/c I am worried he might be uncomfortable on the hard plastic now that's been mentioned.
We have a potty seat that was from my niece (meticulously cleaned, haha) and he sits on it fully clothed for about 2 seconds and he likes to play with it sometimes. I haven't really done a lot of sitting him naked on the potty though, b/c then he just runs all over the house naked and doesn't want a diaper or clothes on again.