Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Jameson woke up from his nap and is squinting in one eye.
Is he comfortable otherwise enough that he is eating, does not have any hives and his face isn't swollen?
We can see squinting in dogs with an allergic reaction but generally we see swelling too, and hives and they are itchy. Sometimes if the allergic reaction is severe they can vomit, thus they are nauseous and don't eat well.
If there is no evidence of swelling or hives then my concern would be that he may have gotten plant material or debris into his eyes or had an accidental paw to the face today when playing which can lead to a corneal abrasion or scratch. Ideally he would see his veterinarian and have his eyes stained to look for a scratch.
I understand that may not be possible tonight.
I do recommend that you flush his eyes several times with sterile saline. This is the same solution that soft contact lens wearers use in their eyes, ask the pharmacist or drug store clerk for help if you are unsure.
The apply artificial tears every hour or two tonight. You can find these available over the counter at the drug store too. The more lubricated his eye is the better for him and the more comfortable he will be.
Keep him in low light as that will keep him more comfortable.
If he is rubbing his face or area around the eye then place an elizabethan collar (cone of shame, lampshade collar) so he cannot further traumatize his eye.
In case this is some sort of allergic reaction you can give him Benadryl (diphenhydramine) at a dose of 1mg per pound of body weight orally every 8 hours, that's one 25mg tablet per 20-25 pounds of body weight every 8 hours. Antihistamines may make him very sleepy which would actually be good.
He should be much more comfortable in 24 hours but continue the treatment for at least 3 days even if he seems much better in the morning and have him checked by his veterinarian tomorrow is possible. If he is not much better in 24 hours or starts pawing at his eyes or he has a green or yellow discharge or bulging eyes he needs to be seen by an emergency veterinarian.
Let me know if you have any further questions.