You cannot keep him afloat.
No one can.
You'll drown yourself if you try.
Keep yourself afloat.
Do things for you.
Make you happy.
Tell him about them.
Include him if he'll be included.
Talk to him.
Talk to him about simple things;
About beautiful things;
About curious things;
About mundane things.
Let him know you're there and floating next to him
That you will help him float if he needs it
But don't you dare drown yourself holding him above the water while you can't breathe.