So if it is a parasite how much would the treatment run? I haven't seen the dog scratching but it doesn't have a lot of hair and has a lot of little bumps on it's skin.
You can rid the parasites cheap enough, the follow up to keep his immune system strong enough to build to keep them away might run a little pricey. Over 6 months my bills must stand at over 10,000Rd but she had alsorts of other problems, lungs, heart etc. Loads of X-rays etc.
Your sound like a allergy to be honest, that could be as simple as casting you 250pesos a month for 6 month to a year. He should be strong enough and well enough to cope by then.
My advice would be to get his loads taken, get him some ointment, cost will be around 350-450 pesos. See what can be done then make a decision. One thing you can't do is kick him back onto the street after you've shown concern.
Run with it fir a few weeks, if you can't handle it then look for someone who can.