Yeah, definitely call your vet. Advice over the phone is free.
In any case, since you know this only happens when he doesn't have a flymask on, I'd just make the effort to have a mask on him whenever he goes out to the pasture. Yknow, to cut off whatever may be going on before it happens.
With my girl, every sunny summer (this also happened a few times during the fall) day that she didn't have a flymask on, I'd go out to see her and find her with "tears" running down her face and flies all over those tears. She'd also rub the areas where the tears ran and sometimes she'd end up with little sores where she had been scratching.
Anyway, I'd get a flymask on her and the tears would cease. I always figured that the flies were the precipitory event - like, the flies irritated her eyes, causing the tears - but now I know that wasn't the case.
There were a few times where her eyes would appear slightly swollen but the swelling would "disappear" with the tears so I figured they were interconnected.
Now, since she's been diagnosed and wears a flymask on everyday where there's a chance for sun, I've noticed that sometimes her eyes will look slightly swollen if I'm a little too late with the mask but it's not a big deal and the mask clears it right up.
Anyway, call your vet. He/she will probably know exactly what to do. :)