He's shown he has no self-control and no respect for you or others.
His motivation in telling you was motivated by either guilt or a desire to be out of the relationship. Both are unwelcome.
If you forgive him this, his behavior will not be curbed, and as he gets older (as he sounds incredibly young), the likelihood of physical straying will increase. That lack of self-control can manifest in ways of poor partner selection or unprotected sex, which can greatly damage your quality of life, if not destroy it entirely.
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