Hi,
As an electro-geek who has installed dozens of after market systems in autos, and has built dozens of closed, vented, bandpass and hybrid loudspeakers for home and auto use, AND who has also worked with teens in mental health and social services settings, I think I might have some info to offer here.

I think you might consider a bigger picture here, rather than just how much current draw some "amplifier" made in china might draw. First, while ANY sub application in an auto will add fill for the lower octaves, (You can basically count on at least 12 dB of cabin gain from ANY sub installed), is your son looking to construct a 6" driver-based folded horn or passive radiator speaker driven by 50- 75 watts? No, your son's choice is 150 watts RMS per channel bridged. What will he drive with the 300+ watts RMS he has on tap? Whatever it is, it certainly will provide considerably more musical "material" for him to enjoy!...

Having said that, the bigger question is, do you really want that much more distraction in the vehicle for his hormone-driven, adrenaline-fueled, adolescent mind? Especially if you consider the other inherent distractions, AND passengers (including-potential candidates for meaningful, but short term relationships), some redbull, 4 Lokos, or starbucks espresso (pick your flavor), some swisher sweets, and a bangin' 105dB stereo blaring in (WHOA) a mercedes benz!... I think you get the picture....

At 16, maybe you should consider encouraging him to save his funds to "trick" his own vehicle, that he can purchase when he gets a little older..