What should be done about a Red Eared Slider with clogged nostrils?

I have a young red eared slider who was lost for almost four weeks. Our neighbour found him caked with mud. After letting him rest in low-level, warm water for a few days, I returned him to the regular turtle tank. In both tanks, he didn't seem to eat at all. Upon closer inspection, I realized his nostrils seemed clogged. There was a tiny opening, but he couldn't breath out of it. His nostrils were almost invisible. His throat seems to inflate for a bit before releasing the air through his mouth. He also isn't eating anything. I fear that he could have a respiratory infection. However, there is no mucus or liquid coming out of the nostrils when held upside-down. He does not have lethargy because he scrambles onto the floating dock quickly. I have relocated him to the low-level water tank again. What could I do to help him?