Keeping the cage secure will be a big issue. Also, dispel any misconceptions he has about snakes (they're not slimy, not all venomous, not all aggressive etc). You could even show him some reptile stats (such as the fact that you're more likely to be struck by lighting, hit by an asteroid, or be blown up in a fireworks accident than killed by a reptile. Really, look it up).

Tell him all the positives to owning a snake as opposed to another animal. They're totally silent, rarely smell, hide and sleep most of the day, only eat once or twice every week or two, keep their mess contained, and the species you want is relatively slow-moving so escape is highly unlikely. Snakes are without a doubt the easiest and CHEAPEST pets I have ever owned, lol.

Or, if you're really really determined, do a full-out powerpoint presentation on the species you want (or even on snakes in general) and present it to your dad. My friend did that to get her chameleon, and it worked. It shows how passionate you are about getting one, that you're hard-working and diligent, and that you know everything there is to know about the animal and its needs, so you're not getting into more than you can handle.

As for species, go with the ball. BRBs are beautiful, but not usually recommended for beginners because they have very high humidity requirements and have a bit of a reputation for being nippy. Children's and spotted pythons could also be good starter constrictors, but most other starter snakes are colubrids.