My 8 week old has been doing a great job with breastfeeding but the last two weeks or so he has not…

My 8 week old has been doing a great job with breastfeeding but the last two weeks or so he has not been opening his mouth enough and it is really hurting. Any suggestions on how to get him to open bigger? I've tried to use my thub to pull down his chin but I can't pull it down.