Ditto what many have said. If your ex thinks it is so normal, he should take son and his piddly $25 contribution to the FOOD he eats and have a nice time living with them. Otherwise what goes on in your home is not his business. Period. THat is how divorce workes. If son doesn't like it, try opening the door at random times when the girl is there, or even getting rid of the door altogether. If son damages the house in anger after this, or threatens/bullies you in ANY way, call 911 and insist that he leave. That twenty five bucks is NOT rent, do not EVER say it is. It is a small contribution to the FOOD BUDGET because there are special foods he likes and you don't. PERIOD. If you call it rent he has certain rights including not having you let police into his room (not a right that ANYONE in my home has) and you would have to evict him formally if you wanted him out.
You also might tell the girl that this is NOT a flophouse or bordello and she is NOT welcome to be in a room behind closed doors with your son (esp a room with a bed), and she is NOT welcome to remove her clothing in your home. I might be tempted, if they persist or he brings other girls home, to go in while they are "busy" and remove both their clothes, tossing them in the back yard, then to go tell them, as abruptly as possible, to both get out and he can come back with-o his hook-up-honey at whatever time he is willing to live by the rules of the home. he can come get his stuff in a day or so. But I can be really mean.
I hate to mention this, but if he has this much disrespect for you, esp with your exh feeding it, you might want to change your sheets often. Sex in a parent's bed has the allure of the forbidden and some people LOVE to do it. I knew several guys who did that to their divorced moms because dad planted the idea or said it was okay when mom was totally against the child having sex, much less at home. Unless caught in the act, the people I knew were quite good at making the beds so the parent had no clue.It is something to watch out for.