Most of the sapling's foliage looks very good and the symptoms visible do not look concerning. It's not likely that the cause of the symptoms is affecting the tree's ability to produce more leaves; rather, it's probably just finished producing its first (or only, for the most part) batch of leaves for this year so is slowing-down the production of more foliage. The deformed and yellowed leaves could be infected with a variety of fungal diseases common to shade trees (anthracnose and taphrina are typical among oaks) but very few are serious with regards to long-term tree health. Treatment with a fungicide is not needed, and since fungicide use could negatively impact other organisms, we generally discourage their application if not needed to salvage a plant. (They also only help prevent infection, as they cannot cure existing infections.) You can snip off the few leaves here that are an eyesore but nothing else needs to be done in that regard. As to why a few of the smaller twigs seem to be still bare and leafless, it's hard to tell as they likely declined weeks or months ago, but any that haven't shown signs of new growth by now are either dead or too weak to keep and should be pruned off. Leave a short stub when clipping branches so you don't make the cut flush with the trunk, which won't seal-over properly.

To support tree health overall, make sure it's not growing too close to lawn (keep them separated if you can by mulching around the tree instead, in a flat layer no thicker than three inches and not covering-up the trunk base). Not only does this reduce it's root competition for resources with the grass, but it avoids accidental injury to the trunk from mowers, weed-whackers, or herbicides, all of which might cause serious untreatable damage. If deer visit the yard overnight, consider protecting the tree's trunk starting this autumn by putting up a barrier like wire caging around it (not so close it touches the tree though) so the antler rubbing of the bucks does not cause significant bark damage. We see what looks like deer netting in the background of the photos, so perhaps you have already taken care of this.

Going forward, you can also monitor the root zone for watering needs to lessen any drought stress as the tree establishes. Feel the soil about six inches down near its root ball and water thoroughly only if it's becoming somewhat dry to the touch at that depth; if damp when checked, the tree likely has enough water without intervention.

Miri

If deer could not access the trunk (and it appears to be lower than they would typically damage anyway), then rabbits or squirrels might be responsible. We can't determine which culprit it was with certainty, but those three are the most common animals causing bark damage of this type.

Miri

You could try using a registered general animal repellent product (such as for deer, rabbits, and/or groundhogs) sprayed on the trunk if you think this will be a recurring problem or are concerned the cage could be moved by a larger animal, but otherwise, there's not much you can do at this point. Some trees are able to deal with bark removal damage and scar-over the wound while others succumb to the tissue damage or an opportunistic infection or wood-boring pest. Their fate is not something easily predictable in these circumstances. Neither circumstance of a possible future pest or disease issue related to the wound should be treated with a pesticide, nor should any type of coating or sealant be applied to the wound, as this could hinder the sealing process. Time will tell if the tree can continue normal growth or not, so for now, it's just a waiting game to see how it fares.

Miri