Need advice please!

hi! I've been with my partner just over a year, we have a 17 week old baby, we don't live together yet as he cares for his daughter during the week. The ex was supposed to have the daughter back full time after I had the baby to help us out but that hasn't happened. The problem is my partner will do everything the ex says, he slags her off to me yet if hes not messaging her hes talking about her! He says they message about their daughter. i understand that but it's getting me thinking he still has feeling for her although he left her as she cheated on him. I love him so much he just annoys me when it comes to me bringing our baby up alone during the week. sometimes for 2 weeks as the ex travels to see her boyfriend. I've talked to him about this but he just says there's nothing he can do as his daughter's mum is next to useless. I have two children (including my baby) absolutely adore them but all I ask is for him to step up and talk to his ex about this! has anybody any advice on what I could possibly do?! sorry for the super long post! needed to get it all off of my chest and this isn't even half of it! Thank you 😘