Cats love sniffing and sneering at any and every food you offer them, even if they ate it with great glee yesterday.
T.S. Elliot said it all in "The Rum-Tum-Tugger".
Homogenised milk is bad for cats. Clean water is their drink and buy whatever brand of food you feel you can afford, and stick to it for at least a fortnight.
If he consistently refuses it, try something else, but if he is just pulling your leg, he might just stop if you refuse to pander to him. (Unlikely, but it has been known to happen)
My two like to chop and change between three different menus, but on the days they have been hunting, they are not hungry, so leave the food I serve. Sometimes they do eat it in the course of the following forenoon.
Right now, there are plenty of mice, as well as young birds who have not learned to be wary of cats!
A stray that has survived for two years, obviously hunts, forages in dustbins etc. So, unless you are prepared to accept full-time responsibility for him, it is actually kinder not to accustom him to one kind of food.