Both "He will be coming (now)" and "He will come (now)" express similar expectations about a future occurrence and seem to me to be virtually interchangeable.
But as Armen Ծիրունյան observes in a comment above, "He must be working" is idiomatically quite different from "He must work." The latter sentence simply states a necessity; but idiomatically, the former sentence is speculative. In a particular context, the speculation might take this form:
He's not here when he said he would be, so he must be working later than he thought he'd have to.
There is no actual necessity implied by a surmise expressed in this form. Merriam-Webster's Eleventh Collegiate Dictionary (2003) defines the relevant sense of must as follows:
4 : be logically inferred or supposed to {it must be time}
In this idiomatic usage, the thing that "must" be true is necessarily true only if the underlying logic or supposition is justified.