Most Helpful Guy
One of the most common yet destructive propensities we have as individuals is to "jump to conclusions".
Your narrative states that you saw him 'on a date with another girl'. So could this have been a business associate, relative like a sister or niece? I am only asking this to clarify if this indeed was another 'date'.
So let's say it was. What to do? Imagine for a moment had you been there five minutes sooner (or later) and you would never have witnessed this rendezvous. What would have happened then? Would you be on your 5'th date by now?
Perhaps it is best for you to keep this knowledge to yourself for now. It could have been an innocent encounter. But we don't know for sure, and until you are certain, don't make assumptions. You could use this 'confidential' info as a barometer for his honesty:
YOU: "Have you eaten at any good restaurants downtown lately? My girlfriend says there are some good ones but I'm not sure. I'd love to go downtown. . . yada yada yada."
HIM: "Why yes, funny you should ask, I ate there on Tuesday with my (sister, niece, mom, )".
You can use this information surreptitiously to determine his level of truthfulness. (Since he doesn't know what YOU know)
From you avatar you appear to be such a pretty and sweet girl, and I hate to see anyone struggling in their relationships. If you really really like him, try to sustain that relationship without being obsessive. Eventually it is who we are that will win the day. 😊