According to The World of All Souls, Baldwin was reborn in September of 52BC, i.e when Philippe offered him immortality and a place as his son.
One thing that happened at this time:
It was the last major engagement between Gauls and Romans, and is considered one of Caesar's greatest military achievements and a classic example of siege warfare and investment. The battle of Alesia marked the end of Gallic independence in France and Belgium.
Is this where Baldwin distinguished himself, enough to catch the attention of Philippe de Clermont and prompt him to offer Baldwin the chance to join the family. He wasn’t injured, dying or ill, he was perfectly healthy when he agreed.
This meant that he had just been through a battle and jumped at the chance to get involved in more. Out of the 60,000 to 75,000 soldiers, Philippe chose Baldwin.
I have to wonder how that went down because we do not know from the books!