Not a rein with a loop in it, you need to keep a contact on their mouth, but ask them to start stretching out into a longer rein. It's tricky if you go straight into a long rein and expect the horse to stretch into it, so start with a shorter rein and get them working their back, so leg, transitions, leg yielding, etc. until the horse starts to work it's back and reach for the rein. Give the reins out a little and if the horse stretches out to reach them (while still carrying itself and not rushing forward or falling on the forehand) you know you're on the right track, until you'll be able to give the reins right out almost to the buckle and have the horse stretching to meet the contact with a loose, swinging back... THAT is long and low and will assist in building up your horse's topline.
Also, what do you feed him? If he's only on hay, it's a bit harder to build muscle, so put him on a good quality, non heating hard feed ;)