What a great goal for the summer---to work with your child on listening skills! One technique is to have the child listen to you for a short time and then have them repeat back what he heard you say. Then increase the time over the summer. Memorizing something fun would be an excellent way to have your son focus. Song lyrics or a favorite poem-- nothing too complicated.
The best way to teach a child listening skills is to model them as a parent. Give your child eye contact. Let him know you heard what he said by using reflective listening. Repeat back in your own words what you heard him say.