You should proceed by not having anything further to do with this guy.
He has shown more than enough signs that indicate that he isn’t someone to be trusted.
He has never paid back the $600 that you lent him.
Ever since he received that money, he has acted distant and barely contacted you and you have now discovered that he is back on the dating app.
Though you have been dating this guy for six months and have most likely developed feelings for him, you need to judge him based on his actions and not on any false promises he may have given you in the past about his aspirations for this relationship.
If you were to stay on in a relationship with this guy, there would only be more and more deceit to follow.
Take this as good fortune that you have dodged a bullet.
It would have been much worse if you were engaged to be married to this guy only to experience everything that you have so far.
Though you may be very hurt, try to avoid using that as an excuse to reach out to him in order to get some clarification.
Again, judge him by his actions.
His actions have spoken loud and clear that he is not to be trusted.
Hence, it is best to proceed by letting this guy go and consequently keeping him out of your life.