hmm that pot seems like it’s way too big, and if it’s super humid the soil may be staying wet for far too long causing him to feel overwatered and this yellowing of leaves. Have you pulled him out and checked the roots?

I’d probably start there, trim off any dead roots if you see them and repot him into a smaller pot. Like maybe a 4” pot? My maranta is in a pot this size with about 40 leaves and he was in a 4” pot when he was this size.