Help your son to recognize the effects and attributes of God, through answered prayer, creation and Bible prophecy.

Heb 11:1-3
Now faith is the assurance of things hoped for, the conviction of things not seen. 2 For by it the men of old gained approval. 3 By faith we understand that the worlds were prepared by the word of God, so that what is seen was not made out of things which are visible.
NASU

Heb 11:1-3
What is faith? It is the confident assurance that what we hope for is going to happen. It is the evidence of things we cannot yet see. 2 God gave his approval to people in days of old because of their faith. 3 By faith we understand that the entire universe was formed at God's command, that what we now see did not come from anything that can be seen.
NLT

You can also talk with him about other things that he knows are true, but he cannot see, like the wind, or electricty. He sees the effects of them, but does not see the substance.

Deut 6:4-9
4 "Hear, O Israel! The LORD is our God, the LORD alone.   5 And you must love the LORD your God with all your heart, all your soul, and all your strength. 6 And you must commit yourselves wholeheartedly to these commands I am giving you today. 7 Repeat them again and again to your children. Talk about them when you are at home and when you are away on a journey, when you are lying down and when you are getting up again. 8 Tie them to your hands as a reminder, and wear them on your forehead. 9 Write them on the doorposts of your house and on your gates.
NLT