I can tell you, from experience, that best friends make the best life partners too! I think it's because the qualities you desire in each are the same. Best friends are trustworthy, dependable, considerate, compassionate, supportive, respectful, and most of all, they love one another unconditionally.
Now, your real issue is that he already has a girlfriend that he claims to love. Another possible problem for you is that if you decide to tell him your true feelings for him now and those feelings are not reciprocated then you are risking losing a great friendship (especially if the girlfriend finds out and starts to view you as a threat). Truth is, even if he were single and wanted to take his friendship with you to another level ~ you two would be putting that friendship on the line because there are no gaurentees that it will work out or that it won't end badly which would damage your already exsisting friendship. Just know that this is one possible outcome but true love is about going out on that limb knowing that it's worth the fruit you're reaching for.
I was willing to take that chance with my now hubby and it has paid off tremendously! We have been together happily & faithfully for 19 years and are celebrating our ten year wedding anniversary later this month = )
My advice is... continue to be his best friend which means being considerate of his feelings and respectful of his relationship. Continue to support him by being there for him especially if he is falling for this girl because he may fall too hard too fast and if he has a rough landing then you will be the first person that he'll depend on to pick him up and dust him off. Personally, I think the girlfriend is too young for any serious commitment and the chances of their relationship lasting are probably slim to none. Timing is EVERYTHING! When the time is right, you can share those intimate feelings with him and hope that he was waiting for you to profess your love!
GOOD LUCK =)