Hello @rOLFCP , usually opening attachments in any context is dependent on having software available on the device that supports that file type. You have provided very little information to assist you in troubleshooting, such as Operating System, Desktop app vs Browser app etc. Nevertheless, here are some things you may try:
If he is using the Teams desktop app, open Teams in a browser (teams.microsoft.com) or visa versa, if he is using it in the browser, download and install the desktop app
Close Teams (desktop app) completely and start it again. When closing Teams like other programs it does not actually close - it continues running in the background, so to close it completely, you need to go to the Show hidden icons button on the task bar, right click on the Teams icon and choose Quit.
Restart the laptop. Often laptops are kept in hibernate state and seldom restarted. Restarting it may sort many a problem.
I hope you are going to have a good 2021 and your son and his school will find Teams useful in their remote learning. Remember, we don't work for Microsoft, we are simply community members trying to help .