he may be 5 years old, but he lived in hell. You grow up fast in hell, since remaining a stupid child isn't an option. His job for the last couple years has essentially been to read and judge his mother's emotional and mental state and comfort her. Additionally he had to do that while acting around her and everyone outside of the mansion. He is probably more capable of suppressing his emotions and putting on a fake smile than any adult at this point (I do not know anyone who can put up that act 24/7 for more than a few months, but he didn't break for at least a year)
Point is, he is well versed in reading the mood/atmosphere and acting accordingly. Combine that with the fact that he has been deprived of love directed towards the real him this entire time and you have a Julian who would die for his love towards the one who saved him