The only reason that Konohamaru was able to learn the Rasengan at such an early age is because Naruto simplified the technique by using shadow clones.
As for multiple shadow clones, I've only seen Konohamaru do a handful of clones at once (and the OVA doesn't count). He does not have the chakra reserves to do the full technique.
I'm also fairly certain that the reason Naruto sucked at ninjutsu in the first place is because of the Fox. Up until he gained control over the Fox's chakra, Naruto's chakra was constantly struggling against the Fox's. Plus, Orochimaru placed the odd-numbered seal on Naruto which made it even more difficult for him to control his chakra. The bottom line is that Konohamaru will never be in Naruto's league. He doesn't have the chakra reserves to train on the same scale with shadow clones. Plus, he doesn't have access to the raw power of the Kyubi.