Yes, but he can't be half dragon and half human. He can only be dragon or human with a few cosmetic changes. If cosmetic changes create a new race, then blond humans are a different race than black haired humans and once you throw in all the other cosmetics, there are 10s of thousands of different races of human.
To me it makes sense, that
magical traits happen while leveling.
For example, in Norse texts, two trǫll (whether risar or þursar or mixed) can have the same two parents. One sibling might be normal human size, shy of two meters, while an other is over ten meters. The difference mainly depends on which natural feature is manifesting the quasihuman shape, such as a mountain stream tending to take a human size shape while a tall mountain cliff might be very tall. The shape is a product of magic − and the more powerful a mage is, the more wondrous a manifest shape might become − such as extreme height.
So in D&D, if a playable Giant character starts off with normal Human size at level 1, but then gains a size category at levels 8 to Large and 16 to Huge, that is fine.
Similarly, the images from various runestones depict the lifecycle of a dreki (dragon). It is born as a normal snake, the european adder. But soon the newborns sprout horns (where the v-shape pattern on the head of the adder becomes actual v-shaped horns.) When larger the adolescents gain arms with hands, prehensile eagle claws. The adult females then have their own brood of adders. Typically the drekar never have wings, but there are known examples of very ancient individuals who do form wings (eagle wings), and depictions of them. Presumably, the dreki has shed its skin between each shift in shape.
So in D&D, if a playable Dragon character starts of wingless at level 1, but gains wings at level 8, that is fine by me. It is precisely the increase in magical competence that allows the more powerful form to happen.
The only nonmagical species is the Human. Everyone else has magical traits − even Orc darkvision. Even Halfling "luck" is magical. Some Humans are magically Lucky, but most arent.
Cosmetics do not create a new race.
What matters most is the character concept. An auxiliary concern is mechanical balance. The least important consideration is trying to enforce "racism" mechanically.
There are many different kinds of "elf". The Norse texts across passing mentions pieces together a fairly detailed concept of an "elf" that I want to make sure is doable in D&D. But Scotland witchtrial records present a fairly detailed concept of an "elf" that differs in important ways that I want to make sure is doable in D&D. Meanwhile Tolkien invented his own fictional elves. D&D can do these too.
The mechanics are fluid. An essentialist, historically racist, approach to mechanics is often wrongminded for magical creatures, and can interfere with reallife cultural concepts.
What matters is the character concept − then D&D 2024 offers several ways to express it.
Consider the Norse trǫll. The risar and þursar can be extremely differnt from each other, but they are all the same "species" kyn, and sometimes even members of the same family.
Magical traits are inherently individualistic and fluid, and nonconducive to "races".
I get what they are saying. They are saying cosmetic differences make a new race. As I point out above, that's a very dangerous position to take. Think of how many throughout history have used that idea to inflict horrors on others.
I mean, it's literally the opposite of unlimited. You are either the mother or the father since cosmetics don't create a new race.
I am talking about "species" of magical beings.
DNA makes sense for the Human species. It makes less sense for an ethereal Fey spirit, or astral Celestial construct.