I have taught all of my horses by adding leg as Jan said and then lots of inside leg and little pulses (a squeezing motion) with a giving inside hand. Eventually, they bend and naturally stretch out. To keep them there I just keep the leg on and if they bump out of it just start gently squeezing the inside rein (make sure you hold the outside leg ready in case they try to throw their shoulder too much!)
In order to make sure he is carrying his neck, you need to start with impulsion. When you have the forward movement from your legs then you can ask for the horse to lower by keeping a light and constant contact, playing on the rein-making sure not to be pulling backwards or keeping prolonged tension- so that you pull his lip upwards. If he starts leaning on the bit,do a half halt. Hope that helps!