Use 'sidetrack' to describe losing focus on the main subject because of other interesting topics. He often sidetracks into stories about his travels.
Pro Tip 2/3
Intentional Shift
'Sidetrack' can suggest someone is deliberately changing the subject. The politician tried to sidetrack the conversation from the scandal.
Pro Tip 3/3
Temporary Detour
When 'sidetracked,' the subject is usually revisited after the brief diversion. Our discussion was sidetracked by gossip, but we got back on topic quickly.