OK I am in the minority here but I can see some of this from Will's point of view.
He has always felt like he was not good enough for Sonny and a little backwater kid compared to what Sonny has done so far in his life. Sonny had been all over the world and had "grown up and was a Man". Will was barely out of high school/college when he came out of the closet and got married to Sonny.
Did Will go about telling and talking to Sonny badly, maybe, but he was looking at it like he finally can be a respected person and measure up to and be an equal adult earning a living too like Sonny and not be the "kept spouse" and he has a chance can earn a lot of money for his daughter's education.
The other day Sonny did say he thinks of the baby as his own too, even though she is not. Will talking to Gabi on his own was, I think, what he should have done, it gave them a chance to talk privately about their daughter. Some conversations should be private.
Now if Sonny was that upset he should be saying something to Will and not pretending he is happy and good with it all.
He told Will to go and take the offer after all.