Spanish is my second language even though I'm far from saying I'm "fluent." I would be the opposite as your BIL. You might not even realize I have a stutter speaking English, because I can change my words or phrases to find the path of least resistance, if you know what I mean... In Spanish I probably only know one way to say something. Also, words like "como" or "cuando" with that hard k sound are hard for me to start a sentence with. Maybe for him, English sounds are generally a little easier. Either way he should learn to be as fluent as possible to have all the options available to him to say things in different ways.