Seeking advice regarding a cat with casual but seemingly persistent diarrhea.

my 2.5 year old cat has been having questionable poops on and off for about a month now. about a week ago i noticed it was super runny, so i went out and got flagyl but then when i read about how horrendous the experience is giving it to cats (horrible taste, mouth foaming, general emotional disturbance for both human and feline), i figured i'd keep an eye on him for a day and see what happens. his poops leveled out a bit, they weren't coming out perfectly formed, nice and solid -- they were still soft -- but it was an improvement on the runny/lighter coloured stuff.

how long can this go on? he's acting completely normal otherwise, eating well, playing, drinking water. and he's still going in the litter box. i tried a new food with him and no change (also i have two other cats and they're poopin' fine).

i just want to know whether it's worth putting him through a week of forcing pills/liquid down his throat, or if it's possible that maybe there's another solution here, or just waiting it out...

thanks!