I think I know why he left without saying anything, 1st reason he might be afraid to become like his father, 2nd reason is that he's afraid to tell her about his worries and end up hurting her with his burdens so he just left as he was so emotionally tired, he was building up his emotions and after doing so for a great time he'll forget how some of the emotions work. there's also another 3rd reason which could be the case that he took his mother with him to another country to avoid the problems his father caused and he couldn't tell her about it or the 4th reason is that his father forced them to travel away after which I kinda doubt. sometimes it's hard to open up about things afraid of rejection or getting hurt in the end or hurting the ones we love due to our impulsiveness. 5th reason could be that he told his father to give him some money so he can travel away because he wants to run away from everything that's hurting him mentally and he just left without saying anything which is of course the wrong thing to do while he has a girlfriend, he just wanted to disappear from all interactions and problems but of course even after going far away to another country he's still haunted by how he left her alone, he's trying to show that he's not affected but in fact he is.