I do agree that he can obviously draw and if he plans on going to the apprentice route, good for him.
However, make sure that he doesn't tell a soul in the shop he apprentices in that he's been tattooing out of his house before getting to the shop. That's a really, really good way to create a lot of ill will amongst tattoo artists. They hate when people do that.
And I'm not gonna give you the line they'd give about how stupid getting an unlicensed tattooist to draw on you is. You're an adult, you've probably heard it before and you made your own decision about it knowing your cousin. BUT just keep in mind that an inexperienced tattoo artist who has yet to go through some sort of legitimate training (a la apprenticeship) can put something on you that looks fantastic for a year or two and then starts to decay into crappiness really, really quickly. One of the most important things you're paying for in a good tattoo shop is the tattoo's longevity, which is directly related to the artist's technique.