My bet is close with yours! He will probably use the paternity test to threaten the mc into staying with him or else he'll take the child away. Or his only intention is to take the child as an heir but the mc will not accept being torn away from his son.
Honestly, I wish we had more info about laws governing abo dynamics in this story so we can get a better grasp whether or not the mc has a chance of getting away from this evil dude's clutches.
Wanting to co-parent is one thing. But it raises big red flags when the dude intentionally raped the mc when they reunited just to determine of he was indeed the omega he impregnated. That has to be against the law.
He already threatened the mc about their "forced mating" before, where in fact he is the one who dig his own grave...
So I'm not closing the possibility that he wanted the child on his own by using his FUCKING charm again on the Omega who clearly doesn't want him in their lives...
That dude is crazy and it's getting worse each chapter...
I just really wish the mc won't be swayed so easily by that dude after he forcefully barged into their lives (with a dash of intentional rape might I add) instead of formally talking things through like normal adults. He could have approached the situation in many ways that do not involve SA but nope. It just goes to show how little regard he has for the mcs welfare. What a scary and bleak future the mc will have if he has to co-parent with him.

That dude will definitely bribe his way into the mcs life and the mc has no choice but to accept his offer because he needs to afford the expensive treatment for his child.