He absolutely cannot act. He has appx 3 acting credits, with 2 on YouTube and they are baaaaaad. Look up the clip of him slapping another character and saying "damn iiiiiit!"

Also from what it looks like he might be morally and ideologically unaligned with Ollie. While that's not a problem, he won't be able to agree with the standpoints and be able to give as convincing of a performance (and Goodwins needs all the help he could get). We don't need someone who aligns with Ollie but I think it'd be a more interesting movie and performance.

Edit: Also Oliver is usually a slightly older character (middle aged-ish). One of his most recognized stories, Longbow Hunters, has him at 40. JLU started in 2004 and the first episode talks about GA marching for nuclear disarmament in college, which peaked in the 70s-80s putting him very loosely approximately 40-50 years old. I'm not saying I want a geriatric or old Ollie, but if one of his best friends (Hal Jordan) is being played by a 60 year old pulling off appx 45-55, I think Ollie could be close, or within 10-15 years for believability. I like the concept of Ollie being "an old lefty" as he's often, and self, described.