Have you had the vet out? Back checked? Tack checked? Have you had a farrier's opinion? Is the horse in a paddock or a stable? Is it fed anything? What is it's daily routine? How long has it been stiff? Is it broken?
And, please don't type in all caps, it come's across as shouting
sorry about caps, yeah tack is fine and so is back. he´s out now but has been in during winter, he jumps like a stag and is very willing but has never been properly schooled i have him on loan for 3 years and this gonna be a challenge!
If he's not been taught how to carry a rider (i.e. schooled) firstly he will be unbalanced with a rider and prone to run and fall on his forehand. Firstly, slow him down til you feel him come back underneath you more and go as slow as it takes. Don't worry about 'implusion' yet! Then introduce some correct bending i.e. use inside leg to get bend through ribcage, not jacknife at neck/shoulder junction. Look at the scales of training - rhythm (this is also why you are slowing him down), suppleness (the bending) then contact (he will take the contact in the right way if first two are done properly). Then its impulsion, straightness and collection. Good luck.