No disrespect, but I don’t think the adversarial approach is a promising one.
As an engineer, I can tell you, that engineering is all about math, and if he wants to be one he should be motivated to excel in math. Tell him he can lollygag on the rest of his courses, but lollygagging on math slams the engineering door shut just like it would be difficult to try first try baseball when you are 18 and expect to make the majors. He may choose not to be an engineer, which isn’t a bad thing, but he should be made aware of the value of math.
Otherwise, I’m not sure that getting on his case will be “motivating” and may actually hinder his self-discovery of his own motivation. Really, to be successful, he needs to be self-motivated, not motivated by keeping his cell phone privileges.