Hello, and welcome to the Parrot Forums family!
Hahahahaha! If this really were the case, I'd say your bird has a hilariously devious sense of humor. I'll tell you this much. Cockatiels are definitely smart enough to do what you're saying. I had a 'tiel who communicated his displeasure via his water. If a day went by where I didn't get to spend the usual amount of time with him, he would throw the colored pellets from his food into his water, dyeing it red. And if it happened two days in a row, he'd up the ante and straight up poop in the water. (Thankfully, I never found out what would happen after a third day. Lol!) But he only did these things when he was displeased specifically about the amount of quality time spent together that day. No subtlety to my little guy at all! Ha!
But that said, I'm still leaning toward this being a coincidence born of positioning. For instance, even before I'd potty-trained my birds, they had a natural affinity for avoiding pooping on me. Like, if I was sitting at a reclined angle, for instance, and they were climbing about on my chest and such, they wouldn't just poop away. Why? Because birds are smart about where they poop, and avoid situations wherein they might get some on the feathers around their vent area. Basically, the closeness of my shirt to their vent discouraged them from going on me.
BUT... sometimes I would hold them at such an angle that my hand was like a perch/branch. If they got the sense nothing was really below them... boom! Mind you, how were they supposed to know I was sitting and they just wrecked my pant leg? Or the lower portion of of shirt? I don't know if I'm describing this well enough, but I'm saying there might be a difference between the way you hold him and the way your mom and sister do.
As for what to do about it, just potty-train him... which, in part, includes training yourself. (If your bird is flighted, btw, this works particularly well.)
Okay, first, the training yourself part. Obviously, I'm not suggesting you are not potty-trained. Lol! But you need to teach yourself to recognize the signs when Mikey needs to go. They usually do a funny little dance, first. When this happens, you need to be proactive and move him to a bird poop appropriate place. (Having a tree stand or a training perch or something nearby where he spends time with you would be ideal. You need a perch nearby where you can put him the instant you see the signs.) Then, when he goes, you can say whatever word or phrase you want to associate with that action. "Go poop", "Bombs away", "Guano", "Fecal Strike", "Drop it like it's hot"... whatever you want. All that matters is you remain consistent with both the words chosen and the tone in which you say them.
Second part of training yourself is to get a sense of the average time between poops so that you can be prepared to beat him to the punch when the body language pops off. On average, smaller birds might go every 15 minutes, whereas larger birds, like my eclectus, might go as long as an hour between fecal bouts. Now, I know you said Mikey seems to be going every 5, but I'm hoping the interim increases as you both work on this routine.
Anyhow, the association built between words and his action is the first part of training him. Second is building the association in his mind about where are appropriate places to go. So, when you see the indicative body language and place him in the appropriate place, wait until he goes. Once he does, say the chosen phrase and then praise him for doing a good job. (Important: Do NOT reinforce this with treats and such the way you would a well performed trick. You don't want to encourage the idea that he will get treats for pooping. Or you might see an increase in frequency rater than decrease.) A simple, but enthusiastic "Good boy" will suffice. Or, again, whatever phrase and tone you have chosen as your praising phrase.
Eventually, he'll put it together that there are specific places he should go to relieve himself... and that you are not among those locations.
Now, this is where having a flighted bird makes this process really cool. I currently have two birds. Ekkies. Jolly flies expertly well, whereas Maya, despite having her full array of flight feathers, chooses not to. (She was never allowed to fledge before going to her first home, so by the time she came to me, she saw flying as something to be avoided.) So, with Jolly, the process is smooth as silk. I don't even have to watch out for when he's ready to go, anymore. When he's ready, he just flies off of me, lands on the nearest tree stand, training perch, or even his cage if it's close enough, does his business, and flies right back to me. No muss, no fuss.
Maya, on the other hand, I do still watch. She'll resist going while on me, but she'll show off her body language to let me know that it's time. At that point, I get her over to the nearest perch and she does her thing. Orrrrr, if I notice it's about that time, I'll just put her on her perch and say, "Go poop." At that point, she recognizes that I've only put her on the perch to do her stuff, so she'll do her poop thing and then wait for me to pick her back up.
Depending on the bird, teaching this can take time. But, as with any behavior being taught, it's all about consistency and persistence.
