The reasons are plenty. He may see no point in it, he may think he's not ready, he may even think he's not in love (though you know he is). From not being old enough to the golden excuse of not being compatible, he may give you reasons that are plenty.
Before you lose your mind in all this confusion, understand one thing―you need to take things slowly. Use these tips to make him want to commit to you.
Always ask him if there is anything he needs or requires. Tell him that he can always rely on you, no matter what the problem is or what situation he's in.
Show him how you both complete each other and how incomplete both of you would be if this relationship has to end. If there is fear of commitment, you will indirectly help him fight it. Always talk about the good times that you both have shared.
Don't forget a small mention of your efforts too. Show him how the love you'll share is alive in spite of a busy schedule that hardly gives both of you any time.
Show him how you guys have overturned mountains of problems just because you'll had each other. Also mention, how difficult it would be, had you'll not been together through these issues.
A connect is this special string of thoughts and emotions that is felt even when not expressed with words. If you mention the importance of this to him, he will understand that this relationship now requires the commitment it deserves.
Another option is to talk it out with him. Tell him how important a commitment is to you and how both of you have reached a stage where you'll can take the relationship further. Best of Luck!