I had exactly the same problem with my dad. I could see sometimes that he had wet himself after going to the toilet, but he would never acknowledge this and he also refused to change his trousers. The same went for the top half of clothes. I eventually got around this, by either accidentally allowing him to spill a drink down himself or food down his top half. He often would sit with a cup of tea in his lap and then nod off, I just made sure it was never too hot as the last thing I wanted to cope with was burns around that area as well.
After spilling the drink, I didn't give him a choice, I told him he would have to change and he usually accepted this.
The other way I found around this problem was not to give him a choice, I never asked him to change his clothes or if he wanted to do it. I kind of treated him like you would a child, I just told him we were going to do it. I would just get myself all prepared with a large towel, flannel, a bowl of warm soapy water, a change of clothing and I would just tell him we were going to change him as I was about to do some washing and I wanted to add them to the wash. He always complied then and he actually enjoyed having his top half done as he liked his back being scrubbed, so I used to remind him of this, I kind of made it into a bit of a fun game.
Regarding the bottom half I used the towel to preserve his modesty, whilst he removed his trousers etc and I gave him the flannel to wash himself. I usually managed to do this once or twice a week but it's better than not at all and he always said afterwards that he felt so much better.
Occasionally he would still refuse to change his trousers so you have to know when to accept it's not going to be done and when to just take over, you just have to learn to be cunning and pick your moments. You do eventually win. I also always found with my dad that he refused to change or wash in the mornings, but in the afternoons he was more willing as he was more used to washing in the afternoon after he used to come home from work, he used to be a builder and always washed when he came home. So think about what his routine used to be like and try to mirror it.
Hope this helps.
Elle x
