Sounds like an opportunity to teach your son what it means to grow in faith towards God.  When we fall into fussing, complaining and giving up – which we adults can also do, though more often than not, we do it in our hearts – we’re really falling into unbelief.  And the Bible teaches that nothing counteracts God’s grace in our lives more quickly than this.

Why?  We might minimize unbelief, and call it by more neutral names.  But unbelief is a very serious sin in God’s eyes – we are questioning His character.  Our complaining means we don’t believe He is faithful or good or trustworthy.

Show your son Matthew 13:58 – Jesus could not do mighty works in Nazareth because of their unbelief.  If your 5 year old wants God’s power in his life at school time, he will have to repent of unbelief.  Gently lead him through the act of repentance in this area; I’ve found that graciously showing children how serious questioning God’s character is does often bring contrition.

This doesn’t mean your son won’t need reminded of this truth frequently.  I need to remind myself daily.  And I also need other to show me when I don’t see it in myself.

Start out the morning, before school time even begins, with a prayer for help in this area:  Dear Lord, please help us to not question your faithfulness and goodness towards us with  our words and actions today.   You have been so faithful to us in many ways – you’ve provided us with clothing and food and (make your own list of specifics).  Thank you that because Jesus died for our sins, we can ask you to give us grace to obey in this area.  Amen.