My dog's cheek is swollen. Is it pain or an allergy? What to do?

Hello and welcome to Petco Pet Education Center, formerly Petcoach.

It is very hard to tell from the image if the face is swollen on one side or not as we can't see both sides and don't know what his normal face looks like.

That being said, if there is generalized (not a specific or discreet area) swelling of the muzzle, +/- around the eyes, +/- the ears, this is more consistent with a bee sting or insect bite and allergic reaction. This can be itchy as well.

I would look for areas of hives or raised bumps on his body elsewhere.

Ideally he would go to a vet to get a couple injections, one of a steroid (depending on how bad the swelling is and what other medications he is on, steroids can be very dangerous) and an anti-histamine.

Without an exam by a vet, you shouldn't give any human medications to dogs, as the drugs may interact with medications he is on or conditions he might have. You vet might prescribe an anti-histamine (Benedryl is a common product used in the US. You MUST make sure diphenhydramine is the ONLY active ingredient as other human medications that are sometimes in anti-histamines can be very dangerous to dogs. The dose for a dog Hercules' size would be about 50-100mg by mouth up to three times a day).

Other causes of swelling/lumps would be a bump or hit and swelling, infection, he could have something in his mouth, etc.

If it is small lump/swelling and he doesn't seem bothered by it, the best thing to do might be to just monitor it for now. If it doesn't get bigger/more swollen or the swelling doesn't spread, then you can just keep an eye on it. If it isn't gone by 24-48 hours have it seen by a vet.

Best of luck with Hercules, I hope he feels better soon.