There could be several factors affecting the eye. The most likey scenario is he was bitten or stung by a bee and is having an allergic reaction. You can give him benedryl at a dose of 1mg per pound up to three times daily. If the swelling does not decrease he will need to be seen by a veterinarian. He may have an infection like conjunctivitis which is normally treated with antibiotic drops or ointment. The vet may also prescribe a steroid for the eye but he must rule out other possibilities first.
A flourescein stain may be performed to rule out a scratch or abrasion to the cornea or a corneal ulcer. Steroids may not be given if an ulcer is present because it will slow the healing time. If left untreated permanent damage can result or loss of sight may result.
There is also a possibility that your dog has glaucoma. Seen more often in older dogs, glaucoma is an increased pressure of the eye. This is a very painful condition. Signs include redness, enlargement of the eye, increased tearing and pain, aversion to light, and squinting. A veterinarian may consider measuring the pressures of the eye if the condition persists.
Your dog could also have allergies and just like in people allergies can affect the eyes. Causing them to itch or burn. It is during this time a dog may scratch by rubbing his face on the carpet or ground or actually scratching it with his paws causing injury that will need treatment.
If your dog has been running through the woods or tall grass recently he could also have a foreign object in the eye or under the eye lid. We see this often with smaller breeds or hunting dogs.
Some dogs have problems with decreased tear production causing the eye to not have that protective film, called tears to protect the eye from the rubbing of the eyelids or washing the eye of debris. Chronic dryness can result in infection and corneal ulcers. A schirmers tear test can be performed to rule out this condition and a medication called cysclosporin can be started if it exists.
Basically an eye problem or injury really should be monitored closely for either no improvement or a worsening condition. I would use a damp tissue to clean the mucous from the eye and keep it from crusting over. A vet visit should be made as soon as possible, because you just cannot see a potentially serious problem until it may be too late and so many of the symptoms he has fit most eye conditions.
Again, it could just be allergies but I wouldn't want you to risk letting it go, or playing the wait and see game when it comes to the eye.