@Vasia W,
Sounds like he's learning how to be in an adult 'couples' relationship. He can't control, manipulate or guilt you into doing what he wants. Hold your ground (without being nasty when you share this with him).
When and if you're able to schedule/spend more one-on-one time with him (call it dates or whatever) separate from others, that'd help to re-assure him. Your finals and your time with friends are important to you. So is he...but he can't have all your time or at times all the time he wants. However, having such a large group I can understand how a b/f can feel a bit removed, too .
He's going to mature and if he realizes how he's being selfish and correct that, things can go fine. At age 18, he's still growing. He seems to need more re-assurance and perhaps some of that that has to come from within himself.