What Are The Reasons Why A Dog's Muscles Would Tighten Up, As If He Is In A Lot Of Pain?

What you describe is not classic seizure behavior.  Dogs can exhibit partial seizures that are more varied in presentation.  It is possible that your dog is actually in pain for some reason.
Things that cause seizures include toxin, trauma, infectious, hypoglycemia, certain tumors, idiopathic epilepsy, severe liver disease, and metabolic disorders.
This dog needs to be examined by a veterinarian.  Contact your local shelter to see if they run a low cost or income based clinic.  Some clinics also will do payment plans.

It's most likely seizures, and you need to take him to the vet to get medicine for him to control it. Hope this helps, good luck.