Rectal boil?

Hi guys. I have a strange situation. My baby has developed a boil, or abscess on his anus. And as strange as it sounds I am fairly certain it is a result of beginning solid foods. When he has solid foods, it gets large (size of a nickel), red, swollen and you can see the puss in it. Not to be to graphic but is looks like a nasty cold sore right before it breaks through the skin. When it is inflamed like this he strains really hard to poop, even when the stool is really soft. If he goes a few days without solids and just has the Neocate, it seems to shrink down to almost nothing. Has anybody heard of this? What could cause this? The solids correlation could be a total coincidence but I don’t think so. Here is my other concern…should I be worried that his colon and intestines have these same sores?