I need a suggestion for a good kids' movie for an 8 year old boy in Russia who has just started to learn English. Russian subtitles is great, but not necessary. Something simple, but interesting. Thanks!

Finding Nemo is a great kids movie... easily understood even if you don't understand English well...

Mister Roger's Neighborhood would be best, I think, if you can find any episodes. It's geared for young children, and though is simple and not too fast paced, it's also VERY interesting for young children.

Everyone likes harry potter, I suggest the first one. :)

Disney movies are universal.

The Neverending Story. A very happy story of a boy about 8 in a wonderous land of make beleive and happiness. He also sits on a flying dog like animal with a low voice.

I suggest ones that are aimed at around the kindergarten / first grade school level but don't have the feel for it being targeted at that group. It would be paced evenly with strong enunciation of the words but wouldn't, at the same time, offend the kid by having you give to him something intended for very small children.
Perhaps Thomas the Tank Engine or Arthur though, really, anything played on PBS from 1990-2000 aimed for children is probably a good place to start. They're very popular, well-known shows and, chances are, can be found with Russian subtitles somewhere.

Disney movies might be good, because their dialogue tends to be simplistic and straightforward. He also might even be familiar with the film, so it might be exciting for him to see something he already loves, just in a different language!

This is a really good question, and I'd suggest contacting your local library or university. They can probably recommend something for you. If you want, I can put you in touch with the ESL program directors at the college that I work at and they may be able to also help you.