He is allowed to keep it if you are a US citizen, so he is taking the citizenship of his partner (rule 3). However, due to living outside the EU he has to request a new Dutch passport at least once every 13 years (so within 3 years after the old one expired, as passports are valid for 10 years.)
See here: https://www.rijksoverheid.nl/onderwerpen/nederlandse-nationaliteit/nederlandse-nationaliteit-verliezen/automatisch-verlies-van-de-nederlandse-nationaliteit
He is allowed to keep it if you are a US citizen, so he is taking the citizenship of his partner (rule 3). However, due to living outside the EU he has to request a new Dutch passport at least once every 13 years (so within 3 years after the old one expired, as passports are valid for 10 years.)