as previously stated:
T1 debounce and read the pushbutton. turn the mos on and start t2
T2 count 10ms and in the ISR increments the variable Ralph
when Ralph reach 5000, turn the mos

Not what you show (if you need to you can use t0 instead of T2).

I can not even make any sense of what you show, use the above technique.

Erik

PS nevah use registers as communication between ISr and main