UPDATE: The rash is gone
Thank you again for all the support and suggestions.
Just 3 days after i took that photo the rash went away, and its stayed away for a week now .
I treated it with Neosporin and Triple Paste at each diaper change, with Nystatin 2x a day just in case it was yeast.
But what cured it (i believe) was me cutting soy out of my diet. I've reintroduced dairy in the form of 4oz yogurt/day and it hasn't brought the rash back. Taking out the soy is the only thing i changed diet-wise, and that's when his rash went away. We're still having him tested next week though to see if there's other foods he's sensitive to.
DS - 6 & DD 3.5