
As I understood he just wanted to fool around not a relationship, then when his rut came and the uke went to check up on him he marked him, so he's been looking for the omega that he bound, also now that he has a child maybe he can avoid some arranged marriage
Before he had to take medicine and maybe spend his rut alone, but now he can spend it with the omega so, and also he already has a child... So for him is a win-win situation

Hypotheses:
One) at time of FWB, DoYoon was young alpha and wasn't ready for or wanted a relationship.
Two) willing to settle down now.
A) per the Korean synopsis: family inheritance.
And maybe a:
B) possible wish to settle down.
Three) convenience as @ania had posited
Four) Perhaps a lingering fondness for the omega.
Five) Benefits both him, his son, and the omega.

from what he said i’m an earlier chapter, his family were monitoring the mc and their kid because of the medication for hyper dominants (as hyper dominants are super rare) and were close to finding out that siyeon is the ml’s kid. the medication is only produced by his family’s company so that’s why they could keep tabs on them and also how the ml found out about them. it seems he only learnt about siyeon’s existence a couple months before he met face to face with the mc. it seems he wanted to claim their kid before his family goes after them. there’s subtle implications that the ml doesn’t get on with his family due to childhood trauma so i guess he’s not willing to have them treat siyeon like they treated him

For the ruts it's not high priority. Since he didn't seek out Noah for that purpose (he had five years). He had said it was "unheard of" that an alpha has spent that much time away from his mate, but didn't seem particularly harmful for them to have stayed apart. Like there were no backlash or complications from having the separation post matebonding.
As for when he found out about about the child - he definitely suspected it - since the genetic disorder affecting the hyper dominant alphas is rare and only affects hyper dominant alphas in DoYoon's family. I'm guessing he'd known about it for as long as his family had, since a year before, when all this had started (though it could have been any time AFTER that as well). He had waited for Noah to come to him first, but Noah never did. My hypothesis being he's known for awhile. But had waited until Noah can't hold out much longer and their child is suffering from less effective meds w/ strong drowsiness side effects. Which works in his favor more for that "wife-trapping" scenario.

Oh, that part wasn't for you. My suspicion was that he had known for a year.
I know. It is easier because it would be part of the deal. It would be harder to make him have sex outside of marriage. What I meant by not high priority is that it wasn't the reason for the marriage. Not high priority, since it's not urgent.
i’m so confused why does the alpha want him now but didn’t want him before