I am one of those people that believe Kaido is actually a dragon that ate the Hito Hito No Mi, Model: Oni (or something similar). That would probably mean his son Yamato is a dragon.
This then would explain a few things:
1.The reason why Yamato is difficult to catch (needing to send all the Flying Six).
2.It seems quite evident that Kaido and Big Mom are planning a wedding to confirm their alliance.
3.It’d make sense that Big Mom would deeply want Yamato in her family if he’s a dragon, since he would most likely be on eof the three races she’s missing (the other two being giants and whatever King is).
4.Marrying Yamato to one of her daugthers could be the big debt that Kaido was mentioned to owe to Big Mom.
*Theory by Ismoista