Ahem.
If the movie goes in the direction of a young, 9-13 year old Astro to try to stick to it's original roots, then I looked up some of these actors:
click for spoilerHudson Yang

A young actor in his late teenage/maybe early adult years but still with a squishy face suited for our loveably chubby Atom!
Albert Tsai

An even younger candidate matching the age of anime Astro is this cutie pie. He's already been nominated for 'Best Supporting Actor' so why not see how he does as a main? ^^
Brandon Soo Hoo

Another older choice in his very early twenties but still with the face of a baba. He could work extremely well for a 13 year old Astro and looks like he has a similar charisma to Astro!
Ryan Potter

Some may recognise this one! At 19 this boy is on the cusp of a young face that could be easily placed as an early-teenage Astro! Plus with his work in Big Hero 6 I'm sure he's already gotten a lot of popularity!
Now, if the company decides that maybe Astro needs to look slightly older and make him older to please those poor sensitive American parents who can't seem to accept that, yes, Tobio dies, then these two could be good choices!
click for spoilerDaniel Henney

Another popular potential choice! If you didn't adore Hiro then you swooned at Tadashi. He's a little tall but considering the amount of CGI needed for this movie to work i'm sure that won't be a problem.
Justin Chon

With more of a 'bad boy' appearance to him, you can just tell that this man (who's like double my age wth) would look good in an Astro wig (please no).
For my own sake I'm gonna add in one more squishy to the list, however he's more known as a singer but has still done acting before. The only problem would be the language really. ^^
click for spoilerDo Kyungsoo

With an awkward little smile and squishy little cheeks this baba is perfect for an innocent little Astro boy, considering he's also a little shrimp at 5'7" (i think that's small anyway???).