I'd get the vet to check him over, with swelling, there's a chance of an abscess forming, potentially something scratched his eye, an ulcer may be forming.
I've literally just been to see a horse that had a bit of straw/reed or something stuck in the surface of its eye like a splinter - so needed anti-inflammatory pain relief (your willow will be helping a bit here) and antibiotic drops for the eye.
Eyes can go wrong very quickly, so anything that's a problem is worth getting checked out sooner than later.
Suzanne
PS the sheep are very happy munching away at our big field now, except Colin and the older wethers as they're in our wee paddock, until its time to make lambs!