HELP!

My 11 week old son was in ones and had several blowouts so I moved him to twos to see if that would help but it did not. I’m wondering what I’m doing wrong! He is my first but I know how to put on diapers from babysitting and nannying and church nursery. What size diapers should my 12lb baby be in? What brands does everyone use and think works best? I’ve heard different brands work better for boys than girls and stuff like that. I’m just needing information and a little help LOL I’m tired of getting pooped on and going through outfits like crazy!