Why is my autistic 5 year old son (non-verbal), all of a sudden speaking, interacting and following simple commands? He has a cold, low grade fever.

Multiple reasons. The best reason would be a developmental leap in functioning. That would be great, and you could keep doing what you're doing from a behavioral standpoint. Occasionally an alteration like casein removal can cause temporary advancement, but that disappears after a few months. This may hint at a metabolic issue if illness helps in advancement. Make sure you've seen a geneticist and metabolic expert.
Improved prognosis. Without knowing more history, it's difficult to answer "why". I think the association with the low grade fever and cold is likely a coincidence. It is cold and flu season, after all. However, what terrific news. Improved verbal abilities by age 5 have been associated with better long term prognoses in people with autism. Continue th pt, ot, aba, vitamins - something is helping!
Chance. Pdd kids develop in quanta, not always gradually. Don't be deceived to ascribing it to an esoteric treatment. This is the nature of pdd. Sonetimes a cold or other phenomena that stimulate the immune system are associated with improvement. Sometimes they're associated with regression. Keep doing the behavioral treatment prescribed and be pleased with improvement.