Yes, not. He will have to be "created" Prince of Wales by his father, the King. Prince Charles was not created Prince of Wales until July 1958 when he was nearly 10 years old, and his investiture was in July 1969 when he was 21.
Charles, Prince of Wales - Wikipedia, the free encyclopedia
Well, of course... But we can assume the current Pr o Wales to do so, can´t we?
If Charles will ever succeed to the throne, William is at least 40.
The then king won´t hardly wait for some further years to create his 40-something year old son PoW!
When Edward VII came to the throne in 1901, he created his son George, DoYork, PoW. The same when George V succeeded in 1910 (Edwd., PoWales was, as you will know, invested a month after his parents coronation at Caernarfon). Before that, every oldest son of the monarch was always the Prince of Wales, even in childhood.
I don´t know if Edward VII or George V, for instance, waited days or weeks after the death of the old monarch (perhaps being announced after the court mourning?) to create the successor Prince of Wales; but no new King/ Queen seemed to have thought IF they made the oldest male child PoW - it happened, all these centuries, in any case!
Even George I, who knowingly detested his eldest son, created him PoW after his succession to the british throne.
I certainly knew about Charles becoming PoW when he was 10 and his investiture in 1969!
But, other than Charles´case, we´re not talking about a boy here, William is a grown-up family father...
Prince Charles, a traditionalist, will assumingly not be the first monarch since the 14th century to suddenly interrupt the practice I described above.
So, in that respect it is, more or less, "automatically".