Sorry I was a little critical before, but you didn't state that you had been working with them for awhile. Sounds like your Dad just doesn't know much about them, I would suggest giving him some basic reading to do about boas (Do you want a boa constrictor???) such as the Barletts book from the library. Generally it is not a good idea to start out with such a big snake, but it seems that your qualified to do so (as long as you've handled big snakes).
My guess is your dad will have a few questions after reading:
1. Where are you going to keep the beast at full size? As I said before, the minimum cage for that size snake is at least 6x3 (maybe slightly smaller, but not much) which is a gigantic cage.
2. How are you going to obtain that much snake "fodder". Large boa constrictors can eat 4 (or more...) adult rats every 2 weeks. Depending on where you buy them, thats at least $6. If you buy rabbits (provided that you can obtain them) that will be on a 3 week to monthly basis.
3. Are you going to consistently handle this animal to make sure that it stays docile and handleable, in case he would have to take care of it while your away, and ect. In my opinion, they are much better behaved than burmese pythons, but all animals have the capacity to "snap" at any given time. If that happens, your in deep trouble. Always have a second person present while interacting with the animal at full size.
My parents didn't really like snakes either, and I had to answer all the above questions. I worked up the chain from newts to frogs to geckos to snakes. Honestly, I would still suggest having something smaller to start with... but thats your (and your dad's) choice.
Good luck
-----
Current Collection:
0.1 Antherystic elaphe guttata guttata - Corn Snake
1.0 Elaphe vulpina gloydi - Eastern Fox Snake
0.1 Elaphe obsoleta obsoleta - Black Rat Snake
0.1 Leucistic elaphe obsoleta linheimeri - Texas Ratsnake
1.1 Morelia spilota cheyni - Jungle Carpet Python
0.1 Tiliqua scincoides intermedia - Northern Blue Tongue Skink