Alex, when you say his belly is huge, do you mean it is suddenly huge and that he appears to have "gained" weight just today? Like is he already too fat, or are you possibly seeing him bloated?
When my colic-surgery filly was sick, when I got to my trainer's barn, she looked like a balloon someone had blown up. I could not get over how bloated she was (happened just that afternoon and was extreme). If this is what you're seeing, then I'd be worrying not so much along the lines of founder, but colic (maybe an impaction -- tall grass worries me).
I've had friends w/ horses who got into the feed room and gorged on grain which was very serious and had to have the horse stand in buckets of either cold water or ice water (I do not remember which now) and dose banamine. This situation was related to worries about founder.
It is a very good idea to put a call into the vet. Given the short period of time, I think (and hope) all will be well and the short amount of time takes my worries for them more towards colic if there's an issue than founder...
Hoping for the best.