I would refer him to scripture, since the Trinity is defined there, but not described in that word. Jesus said that (when He ascended) he would not leave the Apostles “orphans”. By this, he meant that He would send the Holy Spirit to teach and accompany them. After the Resurrection, Christ breathed the Holy Spirit upon the Apostles. So far, you have two of the three. God the Father should not be a problem, I would certainly hope! One God in three natures. You can search any one of the on-line bible concordances for “holy spirit” references and record the chapters and verses, since he is probably not a big bible reader.
Bishop Fulton Sheen used a very interesting analogy to explain the Trinity: Water. It has three natures. First, of course is water in its natural state. Now, freeze it. It is hard and cold, but remains water in its chemistry. Now, heat it to a boil. The vapor produced is hot and seems to disappear into the air, but it is still water in its chemistry. Thus, you have three natures in one substance. Such is God.
You could demonstrate this by heating a glass of water in a microwave it until vapor appeared, then remove the glass and carefully place a couple of ice cubes into it. Then, you would have all three natures for him to see. Dunno, might help!