He got equal inheritance rights as every person gets from his parents. In this case I see it as "You had a kid, you better take charge of that."

Also:

But he will not receive a royal allowance, nor will he be expected to take on official duties, and he has no rights to the Belgian throne. Vandenkerckhove could take his father’s family name, Saxe-Coburg, but has indicated he may not choose to.