The expression tapered nape area seems to fit. The nape (the back of the neck) hairline is different for everyone of us. In the OP's case his nape hairline tapers towards the centre creating a mini-ponytail.
There are three basic styles of rear hairline: Block, round, and tapered.
Here is a woman's hairstyle seen from behind, where the nape hairline is deliberately cut into a triangular shape
Napes (predominantly super-short) are neatly outlined to follow the natural contours of the hairline, beneath either distinctive “weight-lines” of the blunt cut bob, or graduated/ tapered into the soft, feminine curve of the lower neck.
Source: Hair Xstatic
Different types of nape hairlines
Nape hairline. This is the hairline at the back of the neck. Care is needed particularly when designing shorter styles because this hairline can grow into a point at the centre of the nape, or into two points, or into three, or sometimes can grow straight across the nape.
Source: Hairdressing world.com
Image of a boy's nape hairline which has a natural point in the centre.