Years ago in the UK/Ireland when I used to mend 1st hubbys work clothes, I used to use a glue called Copydex to apply patches. I'm not sure what Copydex is, it looks like PVA but smells fishy. It would stand up to many many washes, and the patches are nearly as flexible as sewing. Make the patches round or oval, or at least round the corners and they wear much better. Put a barrier like a plastic bag if sticking over another layer or you will stick your legs together.
I shared this trick with many small holding friends and they eagerly took it up. One guy did complain that there were so many patches copydexed on to his jeans that they would now stand up on their own.
Now in New Zealand with 2nd hubby this trick isn't needed as he wears shorts for work year round!