You’ll need to verify two things before you can know for certain. First, check for eggs. Look everywhere—anywhere your sylveon and your leafeon can crawl into. Second, pay close attention to your leafeon. See if he looks listless, interested in other pokémon, or uninterested in anything else (such as food).
The reason why I suggest looking for these signs first is because when a male gives a female gifts, it’s usually part of a mating ritual, which in turn means mating is soon to follow. Unfortunately, a female eeveelution can only mate certain times a year. During these times, she secretes a specific scent that attracts her mate and entices him to romance her, as it were, right up until they finally engage in intercourse. Once that’s done, the female will find a place to lay a clutch, and once her clutch is set, she stops emitting those pheromones until she’s ready to lay more eggs—which usually doesn’t happen until after her first clutch hatches. During the period shortly after laying her eggs, the male finds her uninteresting, in part because of that lack of pheromones (and thus, a lack of anything to entice him to court her) and in part because his inability to detect these pheromones is a signal to him that he needs to leave her alone with her eggs.
If that’s the case, then you can possibly help by incubating the eggs. Incubation will allow them to hatch faster, and once the kits are born and placed near your sylveon for the first time, this may trick her body into believing she’s reached the end of her mating cycle. Be warned, however. If you do find a clutch of eggs and attempt to move them, you could instead provoke the wrath of both parents. On the one hand, that could result in helping them to bond with each other again. On the other, taking Magical Leaf and Moonblast at the same time is not a pleasant experience.
If you don’t find a clutch of eggs and your leafeon looks listless, take him to the pokémon center and check for any physical problems, just to rule these out. Your local Nurse Joy may also be able to diagnose any mental health problems if your leafeon’s disinterest is actually caused by depression.
If, however, his disinterest is not an indication of a physical or mental health problem but rather a shift in his attention, it’s important to keep an eye on him and try to figure out what it is. It could be that he’s trying to get his mate an even more impressive gift, at which point, you should help him in any way that you can. Or, alternatively, it could be that another pokémon has caught his eye … at which point, you should perhaps inform him that, no, you are most definitely not going to help him in any way that you can.
In short, your leafeon’s behavior could be the result of a few possible things, so it’s important that you hunt around his living space for anything that could be a possible source, then work from there.