I wouldn't do anything. Some restaurants pool tips and some have a percentage divide per hour. Maybe he's supposed to do that. It's none of my business. If a server can't keep a better eye on their tables, that's on them. I don't think any busser would get away with theft for long - like maybe an hour.
OT - I was a server for years. One night I saw a customer pick up my $5 off a table. I followed him out and asked for it back (all kinds of a no-no, but I was mad!) At first he denied it. It was right outside the door, so people were coming and going and could overhear. Then I said, "I'm going to call the police."
He gave it back. Jerk.