At 20 months the molars usually start coming in, and those tend to be the tougher ones compared to the front teeth. Some kids go through it in a few days, others drag it out for a couple of weeks, and the appetite dip is really common during that time. You could try softer foods or chilled purees to make eating a bit more comfortable for him. That said, if the refusing to eat goes on for more than a few days or he seems really unwell, it might be worth bringing him to the PD just to rule out anything else going on.