Diaper rash fiasco (long)

My son is 19mo now and I never had issues with diaper rash with him, until after his father and I split up. Let me clarify, he visits his dad 10am Friday - 5pm Sunday and sends him home with a diaper rash. Every. Single. Time. I've asked to be please be informed when one is forming when he drops son off so I know to be prepared to put cream on and everything. The answer I'd get: "he hasn't had a rash all weekend". 🥴🥴🥴 So I'm expected to believe his bottom got all red/irritated and rashy within the 40min it took for him to get over here? I don't think so. I know for a fact he doesn't clean him properly because he has sent him home on numerous occasions with fecal matter still on his bum, so this is likely the cause of the constant rash. It used to take a day or two of me rubbing in cream for it to clear up. I suspect since he's re-exsposed to the irritant every weekend is the reason that it now takes all week to clear up and it doesn't even clear up fully by the time he goes to his dad's anymore. It's only his butt and occasionally his balls will be red too. Only the area around his butt is constantly red and irritated. Now before anyone says that I need to document and take it to court for neglect on his part to reduce his visitation time or whatever, I am already on that. I document it every weekend he comes home the condition he's in. I will be doing a before as well just to cover all my bases so it can't be said I sent him over with it. Are there any other tips to help try and clear it up? I currently use Desitin extra strength (i think thats how its spelled). Am I just stuck forever trying to take care of this rash with no hope of it clearing up since his father refuses to put cream on it and doesn't clean him properly so it doesn't get worse? Can his pediatrician do anything to help? I feel like I'll just be told to keep putting cream on and there isn't anything else I can do. Could it be something else? Maybe it's just anxiety for thinking it could be something else due to how me and his dad split..he had put hands on me and threatened to SA me as a punishment and I have been having a lot of nightmares lately and I know that part is probably just projection and it is just a rash from not being cleaned at his dad's. I know for a fact it isn't normal because his bottom would never be red like this and then the split happened and the weekend visitation started.....now it always looks irritated and red...I just want my baby boy to not have constant irriation down there

Thanks for reading and I'm sorry it was so long. Any tips to help with the irritation on his bottom when he's home during the week are welcomed.