I am sorry to hear that Jack is squinting.
My concern is that he may have gotten plant material or debris into his eye, rubbed his eye trying to scratch his face because of his allergies, or his eye got hit by your other dog's tail, which can lead to a corneal abrasion or scratch. Ideally he would see his veterinarian and have his eye stained to look for a scratch.
I understand that may not be possible tonight.
I do recommend that you flush his eye 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.
In case this is some sort of allergic reaction give him Benadryl (diphenhydramine) at 1mg to 2mg per pound of body weight orally every 8 hours, that's one 25mg tablet per 15-25 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 5-7 days and have him checked on Monday. 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.
If he is pawing at his eye you should place an elizabethan collar (lampshade, cone of shame) on him so he cannot further damage his eye.
Let me know if you have any further questions.