Don’t break up with him over this one thing. If there’s other bad/weird stuff going on in your relationship, that stuff might be a good reason to dump him, but this isn’t.
Sex isn’t just about a penis going into a vagina; there are tons of other fun things you can do (oral, handjobs/fingering, kissing, dry-humping, roleplay, bondage, sensual massages, etc). If he goes soft mid-fuck, just transition to a different activity.
It would be worth having an in-depth conversation about, though. Ask him if there’s anything you can do (or avoid doing) to help him. Ask him how he’s feeling about it, and assure him that there are other things the two of you can do together. Discuss possible options for alternate activities.
Sexual relationships are about communication and honesty, so talk it out – and don’t get hung up on intercourse because it does not have to be the main event.