Do not use spurs. I repeat, do not use spurs. If your instructor is telling you to use them rather than solve the problem then, I'll have to admit, you might want to find yourself a new instructor.
If he's already dull to your leg, he'll learn to be dull to spurs. Either that or he'll only be responding to your spurs better due to the severe pressure on his side caused by spurring.
I absolutely agree with Phoebe on this one. Although re-sensitising your horse takes some commitment and time, it's all worth it in the end and you will have a much more responsive horse. No more incredibly achy legs! :D
It sounds like he needs to be re-sensitised to you. Sometimes horses get soo used to leg they stopping listening and eventually they can even become desensitised to whips and spurs. I watched a great video on it which really worked for me, I'll link it here for you. https://youtu.be/XdOpPuxg-Iw
Hope this helps 😃
Spurs really aren't that bad and in some cases can be better than using a crop or just your leg. You could use these spurs http://www.doversaddlery.com/m/ladies'-soft-touch-spur/p/w1-c25194a/?eid=X18A00U1000&utm_source=google&utm_medium=PLA&utm_campaign=NB_PLA_Retail_Mid-Atlantic_GOOG&adpos=1o1&creative=182711941560&device=m&matchtype=&network=g . I use them and they honestly don't do anything unless you really mean to put pressure on them. All they do is pinpoint the pressure from your heel that would've otherwise been more spread out.
Use a dressage whip and every time he doesn't react to your leg flick him up, be consistent and eventually he will learn to move away from your leg 😊
Try using a crop instead. Try to keep your leg still. It requires a lot of strength but if you do some exercises ( like posting without stirrups) it will get easier.