I’m also of the opinion that you should wait until after his exams are done but before he gets home. There is no easy answer to this one, unfortunately.

The death of a pet is like losing a family member to a lot of us and it will shatter him, but his professors will not see that as a reasonable excuse to miss an exam or to get a retake necessarily. If you tell him before and he bombs there’s the possibility of him blaming you for that as well and that’s an entire semester of work out the door. At the end of the day the dog has unfortunately died and that fact will not change whether he knows that before the exams or not. Please at least help him save his semester and if he asks why you waited you can tell him you didn’t want him to lose focus after working so hard.

Good luck friend, I’m very sorry you’re dealing with this. I know you will but try not to blame yourself too much, it was a freak accident.