Thoughts on using Dreaming Spanish with child who is still learning native language?

Hello I am a recent uncle! I have been self learning spanish for a several months and took courses in college several year ago now and have convinced my brother to start learning with Dreaming Spanish. He is open to teaching his son other languages, but I want to make sure exposing him to DS before he fully learns english, won't negatively effect his learning of english? If so at what age would be appropriate to put him in immersion environment if neither of his parents are native spanish speakers or even close to it.

Edit: My nephew is currently 3 months old
I am not trying to have him start learning now, this post was meant to ask when is it appropriate? I just know learning a second language early is good, but how "early" is early? thats all guys.