I agree with Anna in that the key is to find out everything you can about him.
His favorite sport.
His favorite TV show.
His friends names.
His favorite foods.
His favorite video games. etc, etc, etc.
Get your ex to help you with this and make a list and write it down if need be. Learn to play the games he likes and watch the shows he likes so you can talk about that when you call.
It may be still awkward at first but after a while you will be into the same things as he is and you will be conversing with him on a different level.
Be patient and give it time.
I am sure your ex will help. It sounds like she wants you to have a good relationship with your son and for that you are lucky.