"He will come back in two hours" vs. "He will come back after two hours"

He will come back in two hours.

vs.

He will come back after two hours.

What's the subtle diff?

He will come back in two hours

This strongly implies that his return will be in two hours from now (the point in time at which this statement is made).

He will come back after two hours

This implies he will return two hours after some instance which has previously been discussed or indicated. For example:

He will leave when he has finished fixing his bicycle. He will come back after two hours

In which case, the time of his return is undefined because the time required to fix his bicycle is uncertain. Contrast this with

He left as soon as he fixed his bicycle. He will come back in two hours.

Doesn't matter how long ago he left; he will return two hours from now.

"He will come back in two hours" implies that he will be back in exactly (or just around) two hours. "He will come back after two hours" implies that he ill return sometime after (could be a much longer time) two hours has passed.

He will come back in two hours

It is 6pm. At 8pm he will be knocking on the door, though he might get back earlier.

He will come back after two hours.

It is 6pm. The earliest he will come back until after 8:01pm, but probably later.