We have a 13 week old puppy and I think he has an infection but I don't know the exact symptoms. Also, how would I treat this OTC?

if its an infection you're going to need antibiotics. but you can give it cranberry juice to drink and it might relieve the symptoms a little.

frequent, very light, urination
possible blood in urine
fever
uneasy – like not wanting to sit or lay down for longer periods of time, wandering around looking antsy
whining (at least with my dog when she had one)

you can call the vet and ask if you can give him childrens buffered asprin, but you'll have to find out the dose for the size. too much will be toxic. and it HAS to be childrens buffered asprin. we were allowed to give it to our dog because she was up all night with it and we couldnt get seen until the following day. it helped with the pain and lowered her fever until we got antibiotics

Symptoms are frequent urination, excess water drinking, blood in urine, and overall discomfort or lethargy.
You can't treat it over the counter, it requires anti-biotics.
If not treated, it can cause kidney failure and death.

Take him to a vet, there is no otc remedy for this, this is the responsibility that you take on when you have a dog, you cannot bypass going to the vet EVER, when you dont know what it is or how to treat it, especially with a puppy.

The urine will be a bright yellow from the blood that's in it. You need to get her to the vet right away if you suspect this. It can kill them if left untreated. The antibiotics aren't expensive for something like this. It will all depend on your area and vet what those charges will be. When my puppy had it, I think the total cost was $75.

Straining to pee, squatting often and producing little or no pee, discharge, licking the area.

They're just like when a woman gets cystitiis.
So peeing little and often or sometimes trying to pee and nothing happening, quite often with a little blood in it too.

If this is the case you will need to take pup to the vets for antibiotics.
Take a urine sample with you to save time as they are likely to want to test it to be sure of correct diagnosis