Dog eats grass

My Boston Terrier eats grass/weeds every time he wants to throw up. He doesnt do this often usually in the mornings but probably does it once a month or few weeks. But I find it odd. He sniffs for a particular kind of weed and eats that until he throws up and then he just goes about his day after. It happens before I feed him his morning meal while we are on a walk in the morning. Anyone's dog do this too? What even is he doing?