STOP feeding him formula. He doesn't need stuffing! Just feed on demand...maybe you won't sleep as long,but that's parenting or you. I'm sure the grassiness will decrease (and your supply will improve) when you stop with the formula. Make sure to burp him extra well in the meantime. You may want to consider gripe water or gas drops if the problem is severe.
Some people need to supplement.
OP, gripe water is your friend.