Sincethen, he's been more cautious.
The underlined part belongs to -

then (noun)
Meaning: that time
Example: 
- Since then, I've been more careful.
- He advised me to wait until then.
- They'll announce their decision next week. Until then, she'll just have to hope for the best.
- We should get there before then.
- They were friends from then on.
- Nothing like that had ever happened up to then.

then (adverb)
Meaning: soon after that : next in order of time.
Example: walked to the door, then turned.

Source: merriam-webster dictionary