Dark green smelly poo!

My baby is having runny dark green smelly poo! And has done since he was about 2 weeks old, I bf for 2 weeks then to sma gold. Ever since the change over he has had green runny poos. he doesn’t poo very often every other day and when he does he pushes and pushes until an explosion comes out. HV says poo is okay but I don’t agree. Help?!