If you're aiming for a date, you can start by putting out feelers. If you don't feel like you can ask him outright for a date, you can go about it in a roundabout way. For instance, you can say what you're up to this weekend and see if he takes the bait.  You could text, "What are your plans for the weekend? I'm going to the movies. U?" If you tell him what you're doing, you're opening the door for him to invite himself along. If you want to go a more direct route, you can go ahead and ask him out over text. Just keep it casual. That way, you give him a chance to back out if he isn't actually interested in you like you hoped.  Try something like "It's been a long week. I could use some fun this weekend. How about it?" You could also be more straightforward: "I'd love to go out sometime. Do you want to have coffee this weekend?" Texting can help build anticipation for a date, so try texting him the day before or the day of the date. Just let him know you're excited about it, or that you're looking forward to seeing him.  For instance, you could write a simple, "I can't wait to see you tonight!" You could also make it a bit flirtatious by adding in a compliment: "I'm looking forward to seeing you tonight, especially in those tight jeans you always wear." If your date was amazing, a text is a good way to tell him that. Of course, a call says even more, but if you don't have the time to call him the next day, sending a text can help reassure him that you had a good time, too.  A simple "Last night was amazing!" is all you need. However, getting more specific doesn't hurt, either. "I loved that you took me to a sushi place last night. It was delicious! And I loved sharing the night with you."

TL;DR in Vietnamese:
Thăm dò anh ấy. Rủ anh ấy đi chơi. Sớm khởi động tinh thần hẹn hò. Nhắn tin sau buổi hẹn hò.