He enters the village to join the tribe completely nude.

Well, there's "At Play in the Fields of the Lord". Nearly all of Tom's scenes after he joins the native South American tribe has him semi-nude, only in a loincloth that covers his groin area. You get to see a lot of his bum, though.
The film is pretty long and gets boring in some places. I should also warn you there's a lot of other nature programme-esque nudity of the natives as well, including children. It's also very difficult to find. But if you can get by all of those, it's actually a pretty well-made movie (besides half-naked Tom Berenger).

I'm a Berenger fan but I haven't seen this movie yet. I have heard it has lots of nudity both front and back. Did you think it was a good movie, worth buying?

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So is that basically all you are looking for, Tom's nude bum?