Only giving him breast milk calms him down and makes him sleep.
Maybe he's hungry?
Babies go through mad growth spurts. They'll go merrily along eating X amount and then suddenly for a week they'll be insatiably hungry, like they've just completed a long bike ride. Could end up eating 2X or 3X food. If he drinks a lot when put on the breast, then that's what it is. If he just chews it a bit, then he just wants comfort and should be cuddled and comforted and hopefully rocked back to sleep.
Hunger = major cause of babies/toddlers waking up, imo. Their lil bodies are flooded with growth hormone at random. Our 18mo goes through a spurt every so often and wakes at 2, 3 or 4am. Otherwise, she sleeps through bar the odd nightmare, and when it's a nightmare you just comfort her and she goes back.
Growth spurts are expected and won't last long 
Just make sure you get as much food down him as you can in the daytime and evening.
Oh and isn't he approaching 6mo? Are you introducing solids? Might need to ramp up the solids to fill him up.
