Jack wasn't completely honest. Being a wizard requires a source of power, some knowledge, and a plan. But having all that, he could do anything. Yes, there are moral implications, not that he hasn't already done every wholesome and wretched thing imaginable with his powers already, in one century or another.
Long ago, he had been scrying across the dimensions in search of power, when he happened across a white dwarf. A star many sizes larger than the sun, but made entirely of white hot matter. Its incredibly hot, and dense, and has plenty of energy for him to manipulate and do all kinds of powerful magics. Time and space fold around the celestial body but it is much less dangerous to approach it from the astral plane. He studied it for years and learned how to use it as power to fuel his spells, each magical exertion cooling the universe he is pulling power from more and more. Over the centuries the star has cooled what it would have over several hundred thousand centuries but this doesn't concern Jack.
"Uncle Jack" isn't your cool 30 year old uncle on your dad's side, not that you realize. Despite his non relationship with you he wasn't lying, mental, memory, and emotional manipulation is easier than stealing candy from a sleeping baby. This is how he got you shipped off to "visit him" for a month.
Jack, has been alive so long that he has experienced every carnal pleasure, as well as every magically augmented carnal pleasure... Like many long lived wizards and witches, the normal ways of feeling good have gotten old a long time ago. Jack had found out a long time ago that altruistic joy doesn't ever stop feeling good. A psychologist might say it was something about connecting to humanity despite how different he had become. Being less emotional, Jack assumed the empathetic part of his brain was the only part he didn't magically burn out, or maybe it was simply some mental deficit on his part either way, he only really felt good when he was bringing true deep joy to another human being.
Jack has found the easiest, most surefire way to get what he needs is to find the awkward teens who hate their bodies and their youth, transform them, party with them, and turn them back and send them home. They want impossible life changes and experiences, and Jack can give it to them, but at the end of it, the kids return to their normal ages and bodies, the adult memories get sealed and replaced with age appropriate memories of watching TV, playing board games or video games, helping in the kitchen, and wholesome "staying with uncle Jack" stuff. When old enough to remember what they really did, the memories resurface but it seems like a weird, yet vivid dream.
Jack quickly lets them have the freedoms they have been denied, bodies they never dared to dream of having, and he watches as his "nieces and nephews" explore a life with no limits. Jack has tried a number of maturity augmentations, varying from none to making the child mentally an adult, but nothing works perfect for everyone the goal is to keep the kid from crying and panicking. Jack has also varied the teens awareness of their changes, and various changes to reality. He has let the costume add to their personality a couple times in varying degrees. Sometimes the transformation is full and instantaneous, sometimes it takes a week. Sometimes its one costume per stay, other times its one per day. Jack can attest, variety is the spice of life.