Clicker training is an excellent way forward for Captain - but youhave to be really disciplined about it, and teach the 'move your head away' cue first before anything else - because clicker trianing is ALL about treats !I thihnk, it's quite agood thing to do with nippers and biters - because you can teach them that the last thing they should do, if they want a treat, is to get mouthy. OTOH, it's a bit of a leap of faith at first, to start really frequently handing treats to a biter !
Simple pressure-release will work for stretches, Nic. The simplest to start with is the nose towards the girth, because if you stand right back by the hip, you can simply ask for it by gentle pressure on the lead rope. Stretch down, between the front legs is ok too, but you have to thread the rope through the legs to get the full stretch, and the direction is a bit awkward. If you're really sharp on the release with the nose-girth stretch, he should soon get the hang of it, and be giving to pressure with other stretch directions.
Another neat stretchy-bendy thing I do, is based on the nose to girth one .. I put my hand on the horse's neck, and ask them to bend around my hand, with the other hand high up on the halter - because I want a bend through the neck, not just the nose rotating up and round. Then move my hand on to the next vertebra, and bend again - seeing how specific a bend I can get round that hand.