I wouldn't have though petco would sell a bird aged wrong knowingly.... then again i don't know petco (none of them in the uk). Make sure your bird gets at least 10 hours of undisturbed sleep a night. With the humping it sounds like hormonal behaviour but he shouldn't have that at 5 months old. gcc get hormonal/hit puberty at 12months/1year (give or take a few months).
Talk to him through the cage, offer millet when he's good (keep fingers away) and when confident try short, gentle scritches through the bars.
When all seems calm, have him out for a bit (an hour at a time or so), practice stepping up with a stick instead of your hand, and when he does bite put him away for 10 min (no attention) then have him out again or at least talk to him, get a toy of a bit of plain paper and play with him through the bars (no fingers).
This is only what I'd do, there'll be others with better advice I promise. AND you don't seem like a bad parrot owner you're just new. Great for you though, you're asking for help and not just hiding him away or giving up on him, so well done you