My 4 yrs old son is diagnosed with short sightedness & astigmatism (-6, +2.25x90) both eyes. Could watching ipad have done it or possibly made it worse?

Not Likely. While there are some good studies from Israel and SIngapore that try to answer the "chicken or egg" cause, most myopia is inherited from the parents. The studies ask "Are kids with myopia that way because they read so much more or are do they read more because they are myopic and don't see as well for distance in sports? Recent research is attempting to see if taking Vitamin B2 (Riboflavin) helps.