My dog has a severe, sudden rash (has been chronic, never this severe)

-He’s half pit-bull, half Australian shepherd.
Low to ground, overweight, back weakness, otherwise strong muscles.

-Rash: chronic; it’s worsened significantly; it has crusts, flakes, and redness/slight bleeding underneath. Most scabs come off as slightly greenish. The itch making him pant and keeping him awake. I included a photo of a part of the rash but majority is on the side he won’t expose to me. Fur falls out from said spots. It’s in separated “oval stripe” shapes from his chest to his lower abdomen.

-Other symptoms: a foot polyp on toe that has recently ballooned; appears to have “fur bits” sticking out at the bottom, irritation between most toes, joint pain that fluctuates, fluctuating bloated belly, a dry nose and paws, and sometimes his gums swell.

-Is it possible he’s worsening for our grass being cut lately? It’s been cut twice in the last 10 days, including today. He always worsens in spring and we can usually manage/help his symptoms, but this is a new level.

-It feels/looks like an allergic reaction, and/or a parasite. I’ve tried de-worming and flea control. I’ve noted improvements with those medications but it always comes back. Should I try either again?
(Both oral medications: Nitenpyram for fleas, and so called “triple wormer” I can’t find the name for the chemicals)

-His body doesn’t seem to fully agree with any food I’ve tried. And/or he won’t eat a food consistently regardless of whether it triggers reactions.

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-Haven’t been to a vet:
*Money is an issue, but we’ve considered using credit and borrowing.
*He loathes being in a car and his joint pain makes it difficult.
*We will again try our best to make it happen tomorrow but for now if you can help us tonight we’d be very grateful.

I’ve given him Rescue and arsenicum 30 tonight. He’s a bit more calm than he was but still unsettled.

*tried other remedies in the past that may have mixed up his system and caused reactions in my maternal panic. I’m learning more and no longer deploying remedies so flippantly.

Please let me know if I can answer anything else. I’ll keep trying to get a photo of the most severe rash section.