For physical games just lend them to him lol
For digital games you have to deregister your own Switch from the eShop profile menu (don't delete your profile as this will delete save data as well) and then sign in to your brother's Switch with the same account. This will make his account the 'Primary Console' and he can download and play any of the games you own. After doing that log back in to your own console and you'll be able to play your games again. There are catches, though:
- You can't play games at the same time - if one of you is playing and the other starts a game up the game being played is paused
- Your own Switch won't be able to play games offline anymore unless you make it your Primary Console via the same method. Any time you start up a game you'll
I did this process when I had a OG and Lite for a brief period of time and it worked well, but of course there was no risk of me playing two consoles at once there lol.