I have no answers or cures for him. But I happen to be struggling with the same thing currently so I hugged him tightly and told him that I get sad for no reason sometimes too. I grabbed my Bible that I had been reading before he came in and read Philippians 4:4-8 to him.
Rejoice in the Lord always. I will say it again: Rejoice!
We can choose to have joy.
Let your gentleness be evident to all. The Lord is near.
God is near us, Baby.
Do not be anxious about anything,
That means we aren't supposed to worry.
but in everything, by prayer and petition,
with thanksgiving, present your request to God.
And the peace of God which transcends all understanding
will guard your hearts and minds in Christ Jesus.
That means that God gives us peace in our hearts that is bigger than our brains can understand.
Finally, brothers, whatever is true,
whatever is noble,
whatever is right,
whatever is pure,
whatever is lovely
whatever is admirable
if anything is excellent or praiseworthy
think on these things.
Baby, that means that God wants us to spend our time thinking about the things
in our lives that we are thankful for, for the blessings He gives us, and every good thing.
Tell me something you are thankful for, something that makes you smile. "I don't know, Mamma. I can't think of anything." Well, I can tell you something that makes me smile. You make me smile. I'm thankful for you. Now it is your turn.
Coffee in my Christmas mug.
That our heater works.
Playing with my friends at recess.
Hugs and kisses.
The conversation continued for several minutes. And then his sweet voice says, "Mamma, I still feel sad." I know, Baby. We will still get sad sometimes. What we need to do is hold on to those things we are thankful for, those things that make us smile, until the sadness passes. And at some point the sadness will pass. Just keep holding on to those good things until the sadness passes.
I don't know how long it will be until my sadness passes, but I will continue to greet the day with intentional thankfulness until the sadness passes. I will continue to hold on to the things that are true, noble, worthy of praise and trust that God is indeed near and that He will indeed guard my heart and mind with His peace.