'There is no way in hell that an impressionable young man would choose duty over him.'
He did, though. He was not so much impressionable as very stubborn and hard-headed.
Despite how he felt about Alfred at first, and it wasn't good on either side, he never outrightly betrayed any oath he made to him. He ran away, he thought about it, he was very tempted, but he never fought against Alfred. Ragnar the Younger, like his father, was very easygoing and forgiving, though, and also respected any oaths he made. Also in the book, by the time Ragnar meets up with him again, Uhtred is married to a Saxon woman and cannot leave either her as well as being sworn to Alfred. Both Ragnar and Brida complain that Alfred trapped Uhtred, which he very cleverly did, but Uhtred sees it as his fate. Destiny is everything.
A big part of why Uhtred always fought for Saxons, aside from being one himself, destiny and being sworn to Alfred, is that as a Saxon he was seen as a lord in his own right. This was not dependent upon Danish charity or recognition but by law. Ragnar once asked Uhtred to swear an oath to him. Uhtred could not, he had already sworn himself to Alfred. But he is quite upset by the very proposition because a) Ragnar is a brother to Uhtred, not an overlord b) Uhtred was an ealdorman by right, and ealdorman only take kings as overlords, not their equals.