That's great! I really think you have a non-issue, then. He already loves to read, reads fluently, etc. Just be supportive when he acts frustrated, to help him grow out of it, and take the rest of the good advice here, and you'll be fine.
I still think that some phonetic drills, if you can get him on board, might help worlds because they could increase his decoding speed. I'm not sure how to increase your child's attention span, but I think it will just naturally increase on its own for reading-- and I suspect that it's not his attention span so much as frustration level that's the problem right now. So I see your problem as mostly managing his stress level in the short term.
The Step Into Reading series can be hit-or-miss quality wise in my estimation, but there are decent books in the series. Some that my son enjoyed at various times:
"The Treasure of the Lost Lagoon"
"The Night of the Circus Monsters"
"The Secret of Foghorn Island"
"The Mystery of the Pirate Ghost"
"The Curse of the Cobweb Queen"
"Tentacles!: Tales of the Giant Squid"
"Monster Bugs"
"Tales of the Giant Squid"
"Hungry, Hungry Sharks"
"Dinosaur Days"
"Raptor Pack"
My son also has a bunch of DK readers (similar style of book), but the levels are different. Depending on your son's reading interest and ability these may appeal more to him. In addition there are several science-oriented early reader series that may be perfect.
If he starts getting more into comics, I can't recommend highly enough the duck comics of Carl Barks. You can find them in large-format reprints for about five dollars each if you hunt around, and each book contains multiple stories. PM me if you want titles of some of the best stories to get you started.
