Hello there, may I help you?
In this situation you son seems to be lacking in confidence the key here is to be patient, remain calm and keep giving positive encouragement, try not to react to the negative "I can't" but reply with "let me help you" and "lets do it together". It you feel frustrated just stop, take a deep breath and remember he will pick up on your frustrations.
Keep praising all the time and turn the negative into a positive. Use example stories to support him and let him know that practise makes perfect. For example when you were young and were learning to ride a bike you remember how hard it was and you thought you'd never be able to do it. Then one day you just got it...just like that, and it was the best feeling in the world, etc etc.
The other thing is also to let him know that it is ok if you fall off, thats part of learning and encourage him to get right back on! Dont push him but practise in small bursts and go back to it. You will be doing a great job if you focus on the above advise to help you son.
If I have answered your question please accept, please pm me if i can help you further.