If you all have really good conversations, then I would tell him the truth. However, if you are not ready, I do know that when new people come around, friendships are made. He may just be interested in the new friends he is making, which is fine.
Stop being a stranger to the new flatmates and get to know them. Do your own thing and create a solid friendship with the people you live with first. Then, from there, I am sure you and him will continue talking, and if you feel comfortable, you can tell him how you feel. Just for now, stop being a stranger to the new guys, since you will be living with them. Enjoy the UK, the new places and people you will meet, and just relax- it will work. No need to get disappointed or confused- don't let people have power over your emotions like that. 😊