He has his dating app set to be matched with 20 year-olds and he is 43?

There's nothing illegal here so just message him and keep the conversation growing. The power dynamic will always be against you. Your chance of success with someone like this is VERY slim unless you plan on just sleeping with him.

Imagine for a second... you want to date a guy who hasn't been born yet. He is more than double your age. He is looking for girls like you for a reason and it is NEVER a good reason.