That stool does look a bit too watery and loose to me. i also note that there is no white in it...Does he get a calcium powder on his food? If the stools continue to look runny, I would check into it further with a fecal test by a vet. But it may be a one time thing, and if so, I would not be overly concerned.
My boy Dezi once pooed a poo so vile and weird, it looked like an alien! I later watched him as he roamed his "lizard proofed" room, and he went over to one corner and ate some thing like a dust bunny mixed with some pet hair, and dead plant material off the floor! Oh yuck. I knew that was why he had such a weird BM. I cleaned the floors more carefully, to get up any hidden particles, and no more alien poo, ha, ha! So, you never know what they have got into. Just keep an eye on it, to be sure it is not a repeated event...