What's the difference between "he's going to start walking" and "he's going to walk"? Are there any shades of meaning here?
Both sentences mean either (A) a decided action or (B) a future happening based on the current situation.
(Of course if you're talking about a toddler, it's not A)
"He's going to start walking" also puts emphasis and focus on the commencement of the act.
Ex. He's tired and he's sitting on the park bench. But he's going to START WALKING soon.
On the other hand, "He's going to walk" doesn't have the same particular meaning and just refers to the act of walking as a whole.
Ex. He missed the last bus. He's going to WALK.
Similarly, if a toddler has never taken a step previously:
He's going to START WALKING.
And if I notice a more advanced (toddling-wise) kid is about to take a step:
He's going to WALK.
"He's going to start walking" sounds to me as a prediction based on present evidence. You see someone about to start walking and you use this construct.
"He's going to walk" could be interpreted again as a prediction based on present evidence, but it can have a different meaning, too. It can be the announcement of a decision he has taken which the speaker knows about.