Yeah, seriously, we're not telling you this to upset you or because we really care where people touch and pet their birds, lol, but because ALL BIRDS absolutely LOVE to have their bellies rubbed, and they usually also all LOVE to be scratched underneath their wings, on their backs, etc...But the problem is that they like it too much, and you're inadvertantly going to cause Lucky to become extremely hormonal...I don't know how old Lucky is or whether or not you have had a DNA-test done to confirm Lucky's gender for sure (you can't tell a male Quaker from a female Quaker visually, only by a DNA-test by blood or freshly-picked feathers), but constantly stimulating his sex-hormones, whether he likes it or not, is going to lead to severe aggression, biting/attacking, constant regurgitation, and then masturbation or wanting to masturbate on you, etc. And what happens is because you get him sexually-stimulated by petting/scratching him on his belly, chest, legs, back, wings, tail, etc., he eventually is going to become extremely sexually-frustrated because he is going to find out very quickly that he can't mate with you and finish what YOU started...That's why they eventurally become very aggressive, violent, and extremely frustrated...And this also can lead to Feather-Destructive Behaviors like barbering, plucking, and self-mutilation (with sexual-frustration and boredom being two of the main causes of Plucking).
***And if Lucky hasn't been DNA-tested and is actually a female Quaker, which is very possible if you've not had a DNA-test done, then every time you pet/scratch Lucky in a place that is sexually-stimulating, you are potentially causing Follicles to form, then Infertile-Eggs to develop, which Lucky would have to lay, with the average size of a clutch of Eggs from a Quaker Parrot being between 4-8 Eggs...And with each Egg that develops and has to be laid also comes the risk of Lucky becoming Egg-Bound, which is 100% fatal without immediate medical-intervention! So that's yet another reason, and a very important reason why you don't want to continue to touch/pet Lucky anywhere except the head, neck, face, and under the chin...That's it. And right now is also a natural Breeding-Season, with the second Breeding-Season being from late August/early September through late October/early November...So again, you're playing with fire if you keep touching/petty Lucky on the belly or any other sexually-stimulating places.
I know it's tough not to want to pet them/scratch them in the areas that they like to be petted/scratched, we all deal with this as parrot-owners, trust me. If you think a Quaker Parrot is tough, try owning any species of Conure, the "Velcro Birds", who regularly just roll-over onto their backs and wait for you to start giving them scritches on their bellies/chests. It's tough because yes, they do like it. Of course they like it, wouldn't you? I would, lol...But it's not just that "Lucky just likes having his belly rubbed", it's not that simple...The fact that Lucky likes to have his belly rubbed by you, so you rub his belly, and to this point everything is totally harmless from both of you. Lucky doesn't initially want his belly rubbed because he's horny, he wants it rubbed because it feels good. That's it....BUT, unfortunately, whenever you rub the belly of ANY sexually-mature bird/parrot, even though it starts-out as totally harmless, IT WILL SEXUALLY-STIMULATE THEM, regardless of whether they are male or female, regardless of their age as long as they have gone through puberty (Quakers go through puberty at around a year-old, give or take a month or two), and regardless of what kind of relationship or bond you have with them. It's simply a natural, biological-response, just like we as people have happen, it's no different. And unfortunately when it comes to pet birds/parrots, them being sexually-stimulated causes nothing but problems, everything from behavioral-issues like sudden and violent aggression out of literally nowhere, to psychological-issues like extreme sexual-frustration because YOU can't "fishish the job" and mate with Lucky, and then most-tragically the phyical/medical/health issues, such as Egg-Binding and death in females.
So that's the way you need to start thinking about Lucky in-regards to the way you touch him, handle/hold him, the things that you provide him inside of his cage, the things you allow him to have access to when he's outside of his cage, etc. Because we don't want you to suddenly one day be petting Lucky and then have him vilently attack you out of nowhere, or have him one day seemingly start "hating" you, or at least acting like he does, and have your bond/relationship with him ruined...And we certainly try to prevent all females from becoming Infertile-Egg layers, because Egg-Binding is a horrible, painful way for them to die, and when a bird becomes Egg-Bound there is little you can do except get them to an Avian Vet immediately upon seeing the bird "straining" and then hoping that they can either give them hormone-injections to cause them to lay the Egg, or get them into emergency-surgery to remove the Egg before it breaks inside of the bird, killing them...
And just as an FYI, if you haven't had a DNA-test done on Lucky (or you didn't get a DNA-certificate with Lucky), then it's a really good idea to do so. It only costs $20-$30, and that way you'll be prepared if any Infertile-Egg laying does start to happen...