There's a mod specifically for fixing biomes that changed in recent updates. https://www.nexusmods.com/valheim/mods/1485

Unexplored landmasses will be updated with the new ores but previously explored areas will not. Although you may also want to consider that with all the new cooking structures you can't just cook in your living room anymore, so you may as well just start over and build a new base from scratch.

Progression feels a lot different with the new foods and upgrades changing when you get stuff. Really is worth starting over in my opinion.

It would depend on how much you have already explored and how far you want to explore to find the new stuff. Already generated areas won't have it, but if you were just hanging around spawn in your previous save, the undiscovered areas will spawn it. You just might have to go farther out to get to them than you normally would.

Then again if you were just hanging around spawn in the previous game you probably also won't be losing much progress if you did just start a new world.

He is only 3 days in, why are you telling him to start over? TC ignore them and continue your world.