So basically your problems are jealousy and alarm at is lack of application in his work/studies.

Why don't you confront him? Tell him that you don't like seeing him slacking off. It's up to him to decide on what to do, but if he's not serious about it, you won't be motivated by the relationship,...

As for jealousy, tell him that him that his problems are not rival men, but himself, that he is by his attitude the worst danger for your relationship.

Of course, depending on his personality, you'll have to more or less sugar coat your message, but you should still be firm about your needs and wants.