It is so stressful when they suddenly refuse food like that, and you and your wife are definitely not alone in going through this. Around this age, some toddlers do go through a phase where their appetite drops and they become pickier, so it does not always mean something is wrong. That said, since it has been going on for a whole month and you are worried about his nutrition, it would really be good to bring him to your PD just to rule out anything and get proper guidance. The PD can also check his growth and weight to give you both more peace of mind.