Well, I'll start telling you from guys
perspective. Some guys have short attention span and like a child
within themselves, they're curious about relationship and love.
However even the best toys given to a child can be easily upstaged
by a better toy few days later. I'm not saying that you're his toy
or he's toying around with you...it's just the easiest way that I
thought I could relate it to you.
It's natural that guys would still want to know more girls...as
much as he could possibly grab hold of. It's like a world conquest
for all the male species...
and it does boost his ego and make him feel more
confident about himself and his present and the life ahead. It just
give a boy a man's confidence, which is necessary most of the time
so that the boys will grow up to be confident straight males.
Hmm...you sounds like he's your first-boyfriend, which is probably
true by the way you reacted. Well, there's nothing wrong with his
interest to know 50% of the world population. (50% guys, 50%
girls... ) Everybody is curious about their charm
and charisma and everyone wants to be the ever popular prom
king/queen of the ball. It's the same for girls too, gals will also think if her
bf is 'The One' for her even when they haven't met any guys better
than her current bf...and that thought most often leads to an
affair and some disastrous outcomes in a relationship. It's
perfectly normal for anyone to have such thoughts at least once in
their life, afterall we're all emotional beings.
You are feeling insecure about his behaviour and you should let him
know about it. Don't worry about leading it into a
quarrel if you can understand what I'm going to tell you.
You're probably upset that he hidden his activities from you, do
not fret...he is afraid to upset you and that's why it was hidden
from you. Right now you're facing a barrier in your relationship
and both of you are still very much learning how to grow your
relationship. You want to know if he still love you? I'm sure he
still loves you cos if he don't, he wouldn't have make the effort
to made you his girlfriend. And you must be really special to him,
else he would've remained as just a normal friend with you.
He still comes back to you at the end of the day,
doesn't he?
Be gracious, although it's a difficult thing to do so...but it's
the most powerful weapon for girls to use against guys if deployed
in the correct manner.
Being gracious doesn't means tolerating his lovey-dovey intimate
smses to other girls. It means taking a step back from this
relationship and looking at your relationship from a wider view
from your perspective. Confrontations are best to be avoided at all
cost, as it doesn't help to improve anything in a relationship
except to strain it.
Listen to what he have to say, and you'll know his
intentions...from a girl's hunch. I can't tell you if he's a bad
guy who planned to cheat you of your feelings and body. But you
know him better than anyone else here, so judge him based on who
you know he really is. Take a time-out from your relationship only
when you're really unsure of his intentions towards you and this
relationship itself. Time-out means temporary separation for both
of you to see the bigger picture of the mess that was once a
relationship.
If he still loves you and treasure this relationship with you, he
will know what to do. He will do anything to preserve this
relationship, yes he will.
Hehehehe...there isn't just one way to let him know about your
frustrations. The best way that I recommend is to get him a card,
and tell him what you feel. Do remember to let him know that you wished that he will
spend more time with you than with other girls. 
(That's evil....:lol And allow the neglecting factor to be the main reason
for getting him a card rather than the other girls issues.
Cards are necessary and when you really need them for situations
like this, it's money well-spent even though you might wondered why
they're being overpriced sometimes. (I'm not a card seller 
)
Good luck.
