GUYS HE'S PREGNANT WITH ERWIN'S BABY!!! YESSS!! SO MUCH YESS!!

He's not pregnant. I think the lactating part is just the way of omega in this doujin. Look at this page http://www.mangago.zone/read-manga/shingeki_no_kyojin_dj_omegaverse_milk/mf/mi_chapter-1/pg-10/

He's going to be after this.
http://www.mangago.zone/read-manga/shingeki_no_kyojin_dj_omegaverse_milk/mf/mi_chapter-1/pg-18/