Help with getting my colt collected...

I have been working with my colt for about 4 months now, just slowly training. (He gets riden about once a week)
He walks and trots very nicely and will collect and give to the bit when I ask him to. He responds to leg yeilds, is slowly getting the hang of pivoting, and can somewhat sidepass. He is a really easy guy to ride, but his lope is a different story.

He can't get collected for the life of him at the lope. Even when he is in the pasture, he runs around with his head up like a carraige horse. I've riden him with draw reins and it helped, barely. Once we lope around for a bit he relaxes, but he still keeps his head way up. He can get really rounded at the walk and trot, but the lope is different story.

Could it be because he is out of shape? He doesn't have a lot of muscle built up in his hind end yet, but he is slowly learning to grow haha:)
What could I be doing wrong, or what can I do to help him?

He's probably not very balanced at the lope yet, I would suggest doing lots of transitions and have him doing all the basics perfect before you consider loping. This includes all his transitions, stopping collected, and turns on the forehand and hindquarters when asked. That will build up his balance and will make it easier for him to use his hindquarters in general.

Ditto to what was said above. Get him comforatble with loping and carrying himself undersaddle first, and if you're still having problems with collection at a lope, I would try a martingale for a couple rides and see if that affects anything.
That's what I did for my 3 year old and I can now collect her in all gaits.

Like mentioned above he doesn't know how to balance himself at the lope.

He may be collecting at the walk and trot. But what I notice some people doing is that can get their horse to flex vertically giving the illusion of collection, but they haven't taught the horse to drive up over the bit, round up and drive his legs up underneath himself. Until he learns to do that he will never really be balanced and collected. You must have a good lateral flex to begin this. With colts I will teach them to trot and give softly to the inside of the circle then pull my outside rein straight back to the hip to add the vertical while kicking him up over the bit and driving him with my seat. When he softens up over the bit and feel him drive up under himself with his hind end(You can feel the difference) I soften my hands and won't drive as hard with my legs for a reward. If he hollows out, repeat. Eventually he will get it. Some horses are bred to do it (I have a colt that I could lope collected by his tenth ride, and one that's 12 years and still doesn't get it)and some never really do it well. Conformation can play a roll as well.