I only just started to try and brush Bailey's teeth, he is 5 1/2 months, his little mouth is so tiny. He takes a fit and acts like I'm trying to kill him when I go to lift the gums to try and brush, I just give up, I can't get his little mouth open with out him crying

What am I doing wrong??, I'm so worried he will have bad teeth, I know brushing is essential, are there any tips?