Racuerex
Hey!! Try to get a empty tin can, put 20 pennies in it and close it up with duct tape. (leave the can lid on to give better security that the tape will stay on) After your dog gets done with any meal, or playtime, take him outside right away. Paise him with a treat!!! and with happy good dog!!
When he decides to go into the house, (only if you catch him in the act of peeing or pooing) Shake the can really hard and drag him outside. Do not do anything else to him. If he pees outside at that time, praise him with happy good dog voices.
I find My Moose needs to go outside 1/2 hour after eating. He can only play hard for ten minutes and needs to go pee. Moose still shocks me once in a while and just pee for the heck of peeing in the house. I know he knows how to let me know, but I think sometimes he wants to let me know he needs to make sure I am still paying attention to him.
I do not know how old your Boxer is, but it could be a medical thing.
Try getting your Boxer outside right after his meals and after playtime. Praise Praise and more Praise. Boxers are very sensitive. They get very insecure when they get punished.
Moose hated the tin can thing, but I never yelled at him while I did it. I let the "bad puppy god" do it for me. Once I got him outside, I waited until he went potty again. I once waited an hour. But I wanted to instill the good dog praise, so he would know that outside was the place to go pee. Giving him a treat works well too. he will associate the treat with doing something special, and he will want to duplicate it.
I hope that will help you.