i don’t think they’re bonded which i’m pretty sure they need to be in order to like reliant on each other pheromones like you said. But i’m really think he’s pregnant because of mornings sickness he’s experiencing
I think even if they're not Bonded yet omegas usually need the father of the baby's pheromones when pregnant
There aren’t really many hard and fast rules when it comes too the details of the omegaverse so I think most authors just tweak it in accordance with what their story needs. I’ve seen bond sickness tropes, couples needing to be mated to bonded, accidentally bonding and phantom bonds.
We all know he’s pregnant by now and are just waiting for the official announcement but I’d just recommend going with the flow for these stories as the details tend to change almost every time
Euihyoung is just Euihyun's brother but I think him feeling sick after being separated from Taeju (he needs baby's daddy's pheromones) it should confirm that he's actually pregnant for real this time.