Congrats! My Green Cheek is the love of my life, they are truly called "Velcro-Birds" for a reason...
I don't believe that you'll know his gender from the Petco paperwork, as they do not ever do DNA testing (many, many Petco Green Cheeks and Sun Conures come through the Avian Rescue I work for, and I've seen many copies of their "paperwork", it's basically their hatch-certificate with their hatch-date, the vendor's information, etc. But they do not do DNA-testing, so if the Petco employee told you that, they were very wrong...But, no big deal, you can order a DNA test-kit online for between $20-$30 where you pluck a few fresh chest feathers and send them in, and in a week or two they send you the results, or you can take them to an Avian Vet and have it done; you should find a Certified Avian Vet or Avian Specialist Vet asap anyway, as they should have a complete wellness-exam within the first month you have them, so you could have them do the DNA test at that time)...
What is his regular, daily staple diet? I'm assuming he came from Petco on a seed-mix, but maybe you got lucky and they weaned him onto pellets, I have seen that a few times, it just depends on the Petco...One thing I want to make sure that you know is that he should be getting lots and lots of fresh veggies every single day, they are great for him and he can eat pretty much any fresh veggies EXCEPT for onions, as they are toxic to birds (as are Avocados)...However, you do not want to feed him Fruit every day or in large quantities. The reason that "he loved" all the fruit is because it's extremely sweet due to all of the sugar it's loaded with. And even though it's natural sugar, it's still sugar, and it gets turned into fat and stored in their livers just the same that the fat from sunflower seeds and nuts do. So he should not be fed fresh fruit more than 3-4 days a week at most, and that should be in very small portions. The number one reason that pet/captive birds die young is due to Fatty Liver Disease, and this often gets blamed solely on fatty seed-mixes that contain sunflower seeds, peanuts and other nuts, millet, corn, etc. And yes, these are very bad for them; however, people don't ever think about Fatty Liver Disease being contributed to by feeding their birds fresh fruit every day...it's essentially no different than feeding them candy everyday. So please try to limit the fresh fruit to being occasional, and instead start trying out lots of different fresh veggies, which they also love. And he does need to be eating a seed-mix, but a very healthy, varied seed-mix that contains no sunflower seeds at all, no corn, no peanuts, but rather a lot of different types of seeds, grains, and legumes, along with dried veggies and other pulses. Seeds keep bird's GI Tracts healthy, as they are designed to eat seeds and grains, and they also keep them psychologically healthy and happy, as they are great for foraging activities. But a healthy, varied, sunflower seed, corn, and peanut free seed-mix should be a daily supplement to a health, fortified pellet diet, and there are quite a few pellet brands that are great, but again, stick to "Natural" pellets and not the "fruit-flavored" pellets, as again, the fruit-flavored pellets are full of sugar, just like fresh fruit is. So any of the Natural Pellets are wonderful, including Harrisons, Tops, Roudybush, Zupreem, Tropican, etc.
I think they get joy flinging food as far as possible 
