I can see where they have got that message from Luke 18:14
"every one that exalteth himself shall be abased; and he that humbleth himself shall be exalted"
i.e if you tell God you are bobbins he laps it up.
However, as you have discovered, the religious are very prone to make thing say what they want to. If you are worried about his self-worth and so on then I would suggest that the Abrahamic religions are not the way. Later on he will learn that no matter how good he thinks he has been, God had to murder his own son to be able to bear to look at him, such is his abhorent nature to the loving, merciful God that made him.
If you are lucky DS won''t then ask quite how murdering his perfect son allowed God to forgive sins if he couldn't before.
If you are really lucky he won't ask why an omnipotent being would need to do anything - if he is omnipotent, who or what made him allow Jesus to be murdered / cause him to be murdered?
The Christian message is - God made us all with perfect knowledge of what we would do, but hates us so much that once he drowned everyone, and then regretted it, even though he is omniscient. Then he waged some wars, slaughtered people, sent plagues, tortured Job (who was sort of perfect too - God hates sinners, but being perfect isn't a great plan either) and generally thought we were all a bit rubbish - so rubbish that he couldn't stand us unless we murdered animals every now and again to appease him.
Then, perhaps sick of all the complaints from the animals, he made a woman pregnant even though that wasn't necessary, and even though he'd deliberately made childbirth painful. Then he had his son tell people that God loved them - even though they still suffered and God could stop it. So convincing was the word of God spoken by actual God / God's son, that hardly anyone believed him at the time. In the end, God had his son slaughtered so we didn't have to kill animals in order for God to be able to tolerate us - his beloved children. He's not done much since. The End.
If you're happy with DS having to make sense of that lot, and having a heavenly father that loves him so much he can only stand to look at him now he's murdered his perfect son (not your DS - the good one, Jesus - that son) then keep sending him to a Sunday School ....
think the message is inappropriate btw :)