All that said, the recoil is really not an issue. Using a formula for felt recoil a 10.5 pound .450 will fell like a 12 pound .500, will feel like a 14 pound .577 will feel like a 16 pound .600.
If this is your first double I would advise you to start with 375 H&H, like everyone else you’re going to shoot your rifle at the range 99% of the time so it’s very important that you get used to handling the recoil without developing a flinch.
See above comment by @cal pappas in regards to feet recoil. Unless he is going to shoot a 12 pound .375 flanged, a lighter gun in a lighter caliber is not going to be a starter gun in regards to recoil.