I guess we just view things differently, they were too young so I can understand him not knowing how to deal with an unexpected son at that point in their lives, but I still think at some point in his life he should have fought harder for his son even if Emily and his parents were pushing him away.
Again we are missing his side of the story but he is not an innocent figure in all of this.
It's stated that Emily avoided him for the entirety of her pregnancy, during which time she was tethered to Andres. By the time she had given birth to MC and returned to college, Santi had graduated and left town knowing that she wanted nothing to do with him. She made her choice and he respected it.
Should he have done more? Yes.
Could he have without burdening the mother of his child with lawyer fees? No. That would be his only choice considering how Emily felt at the time, which undoubtedly would lead to more resentment and a situation no child should be brought up in. So he relinquished and let Emily raise his son as she saw fit.
We'll see what happens when Santi learns about Andres only dating Emily to get back at him, the ultimatum and how he punched his son.