He's eating some solids (maybe a tablespoonfull of some veggie or meat each day) but mostly breastfed. He's been sleeping terribly (like up every hour) for the last week, but not showing any other signs of discomfort or constipation.
Prunes. Or prune juice with water. My dd went through the same thing during growth spurts, every little bit of nutrition in the breast milk got used so there was nothing to poop.