in the book, Chase is going to know Josh is a omega in the end of the book. If my memoirs serves right, Chase is going to be kidnapped. Josh and the others are going to save him and when this happens, the kidnappers does something in Josh's ear and the bandaids in his ear are going to drop off. Chase see the mark and figures out. At first, Chase is going to be confused, them a little irritated, but in the end, it's going to be okay. I remember that his brother didn't have any relations with the kidnappers.
tbh, I believe it's that he chose to be dumb because the other party is Joshua hahahaha. Iirc, the moment he knew the meaning of that same C-scar Joshua has from the another person, he knew there's a child involved in Joshua's life. I believe he chose those things that were convenient for him as long as Joshua will stay by his side.

SPOILER
don’t get your hopes up cause chase is really dumb, it takes a while for him to find out that Josh is an omega and if im not mistaken we’re close to meeting chase’s brother (who is an asshole)