A 405gr at 1700fps is actually the black powder equivalent load that is under the 28,000 CUP pressure limit set by SAAMI for all 45-70's including antique trap door and rolling block rifles. Marlin lever rifle data often goes up to 35,000-40,000 CUP. You can push a 400gr bullet at just shy of 2000fps at that pressure. Falling block rifles like the Ruger #1 are often loaded up to 50,000 CUP which can launch a 400gr bullet at up to 2100fps.You can push a 405gr cast, gas checked, bullet to 1700fps or so using Reloder7 and be well under pressure limits.
I use Remington brass, 43gr Reloder7, CCI 200 primers, and a 405gr hard cast bullet with a gas check. I've run it over my chrony and get just over 1700fps with an 18.5" barrel. Some push them harder but I think this load is hot enough for my Marlins. If I want to go faster I use the Ruger #1.
There are basically 4 levels of 45-70 loads if black powder is included. These levels relate to the different rifles that are chambered for the cartridge with their relative strengths taken into consideration. You didn't specify the rifle so you really need to give more information AND get good reloading manuals.
Other than a ruger single, what lever action models can take the hottest loads?
Assuming your son is shooting a Marlin: I use Remington 405 gr JSP bullets in my Marlin 1895 with 46 grains of IMR 3031. That's for about 1600 fps and is at my "max" of comfort level. They can be pushed harder, but remember, hitting what you aim at and can control for a second shot are at least as important as "punch" in a bear rifle.
Winchester/Browning high walls (dropping block action) and Mauser actions also usually fall under the same 50,000 CUP pressure as the Ruger #1/#3 (also dropping block action).



























