He still is a boy. He is still a boy.
Is there any difference between the two sentences? How about,
He is a boy yet. He is yet a boy.
What are the differences?
Both positions of "still" are acceptable, but after the verb ("He is still a boy") is much more common. (GloWbE has 86000 instances of "still + [be]" against 520000 instances of "[be] + still" (where by [be] I mean any part of the verb 'be' - am, is, are, were, being, etc.)
In Modern English, "yet" is almost confined to negative and interrogative contexts (it is a negative polarity item), so both of those sentences with 'yet' look old fashioned or poetical. I think the choice between them would be governed by prosody and rhythm rather than meaning.