As baptized Catholics there is an obligation to marry in the Church. By not doing so and without proper dispensation, the marriage is not considered valid by the Catholic Church, and the intimacy that comes with marriage can be grave matter, which could then be mortal sin. Sexual activity outside the confines of a Catholic marriage is grave matter, if done with full knowledge and consent it meets the criteria for mortal sin. Mortal sin takes one outside the grace of God and may cause the loss of eternal life.
He can go to confession but without a firm resolve to not commit the sin again, he may not get the absolution needed to be in a state of grace.
You desire to have a non religious outdoor wedding, so yes, it are taking chances with his and your own soul (if you were baptized Catholic).
My best advice is to make an appointment with the priest of your FI’s parish and discuss the issue with him.