What a sweet spangled bubba! I have a thang for spangles. He looks to be a good chunky chicky.... a good attribute for a spangly.
If you are wishing to breed, possible french moult is an issue.... although I agree with the others here that this bub still has some growing to do. Could be a number of reasons why the flights aren't growing in as the breeder expects... nutritional health, seasonal variation, or just the peculiarities of various bubbas.
If you don't wish to breed, possible french moult is not an issue. If Bub is not affected prior to 15d (apparently), immune system issues are unlikely. And most issues relate to breeding.
If this is to be a pet/companion rather than a breeding bird, it's not really an issue if it is FM. And it could turn out to just be slow maturation.
It is good of the breeder to at least flag the issue and give you the benefit of their experience.
If you like the bird (he's a cutie!), go for it.
You can always choose to not breed and just keep him in your flock if he turns out to have issues.
Best of luck and I hope he grows out ok!!!