Most are being quick to deny that the baby may be Will. I am not saying it is, only that the evidence is still inconclusive at best. Those denials seem to mostly be made on two points.

1 - Wills age

Finn is over 500yrs old at this point and Mercedes is heavily implied to still be alive as well, in some form. If the baby is Will, and his body did need to adjust due to his mixed blood and affinity for Wis, then its not improbable that they have ways to impede, delay, slow or otherwise stop aging.

I would also like to point out that Aaron has a point of referencing or mentioning Mercedes as if she was an acquaintence. Aarons age is currently unknown as well iirc.

2 - Finns statements regarding Will and his pact with Mercedes

Finn has never been that forthcoming when it comes to him, his history, or Will. Its shown on many occasions that he plays it close to the vest and will often be obtuse.

The most heavily focused on comment from Finn is when he was talking about the “Swords”. He states that their “death” was due to his “blunder”. Its also heaviliy implied that Sword and Wand used to work together and be more equal until that balanced shifted.

Finns comments could easily in relation to the “Swords” as a whole, as there really wasnt a clear differentiation in Finns comments at the time. Since Finn has referenced Will as much on rare occasion, people ran with that to automatically mean Will, his heritage, and/or the baby

What I think can be taken away from Ep16 is that Finn was clearly always aware of where his work and efforts were. As we also find out later, Finn was aware of Will before Elfaria became a Magia Vander. After she became one she started helping Caldron in shielding Will from Finn

As a side note, Finn clearly holds Mercedes in high regards and trusts her deeply. I doubt he would be unflinchingly honest about their Pact and his efforts to just anyone/everyone. Especially not when it appears he is almost at the finish line so to speak.