Drune's life similarly has had its rough moments and, in turn, has caused him substantial hardship. Seeing him would have been a dream come true and will have shaken him to his core in the best of possible ways. I see him just standing there, mouth agape and eyes bulging. For Drune, this would have solidified a ton for him as he has become rather devoted to serving Narnia and, more importantly to him, Aslan since joining Winterden. I imagine this will spark some changes in Drune and more than likely increase both his devotion and fire for his Country and the Great Lion.
P.S. Still trying to process how he would fully react, but this is what I got right now. XD