You were the anonymous who was concerned about the fact that your leafeon was not photosynthesizing or eating, right? If so, then if you have the suspicion that your cousin is feeding him, then I would recommend being open with your cousin about the situation. Tell them you’re concerned because you’ve noticed leafeon is no longer engaging in his usual feeding habits. Once you establish that you’re concerned about leafeon’s health (and not accusing your cousin of any wrongdoing), simply ask them if they’ve been feeding your pokémon. If the answer is yes, work with your cousin to make sure, for the peace of your own mind, that your leafeon’s nutritional needs are met and allow your cousin to care for him while you’re away. Ensure that babying does not involve spoiling and trust that your cousin is doing their best to help you while you earn your degree.
If you’re also concerned that this may affect your leafeon’s loyalty towards you, it’s true that this may cause leafeon to grow fond of your cousin, especially if you don’t make it clear to him that you’re not abandoning him or giving him away. In order to counteract this, it’s highly recommended that you set aside time, as difficult as it may be, to give your leafeon a proper dose of affection. Insist on being the one to take him to parks or outside, and if he refuses that, be sure to play with him indoors. While, yes, higher education may inundate you with work to the point where you feel overwhelmed, even you need a break now and then, and studies have shown that spending time with pokémon can greatly reduce stress. Even simply an hour will achieve this affect while cementing your relationship with him.
If, however, your cousin is not feeding your leafeon or babying him, there are still the possibilities that your leafeon has grown picky or is stressed out due to a new addition to the household. If neither you nor your cousin have caught new pokémon recently, then consider switching brands of poké kibble. It may even help to make sunning more appealing to him by changing parts of his usual sunbathing spot or routine. Add a bed, place toys within reach, or reward him with either additional treats or affection (in the form of petting, playtime, or brushing—whichever he prefers). In short, tastes and preferences may evolve over time, especially if your leafeon grows accustomed to being babied, so the issue may simply be that your leafeon is telling you he’s bored with his usual routine.
I wish you the best of luck, anonymous, and by all means, please don’t hesitate to come back if none of the above works for you.