Seems like you have 2 concerns. One for the wellfare of yor animals given that he's so young. I wouldn't hold his age against him in that respect (since we have so many first rate teenage keepers here as an exammple), but some of the other aspects you mention do seem suspect, so I think you should talk it out with him until you either feel comfortable with the deal, or decide to walk away.
The second concern you seem to have is a legal one. A 13 year old cant really be held to a contract. He's not old enough. I'd get the parents involved since they are the ones responsible anyway. This is the cost of being a minor, and if he's a responsible and experienced "breeder" as he claims to be, he'll understand that this is a legal formality you need to go through to protect yourself. If he cant understand this, you're better off not doing the deal anyway.
1 adult bull snake: "Dozer"; 1.1 juvenile bull snakes: Oscar and Phoebe; 3 baby red-sided garters; 1.1 macklot's pythons