"If he were an astronaut, he would." ... This sounds educated. Examiners should mark this correct. "If he was an astronaut, he would." ... This sounds casual, and close to slang. It's sounds less educated, but I would personally say it is not incorrect. Because there are times when I would adopt this tone. For example, when speaking to violent simple-minded people - they may be offended or laugh at the "were" tone here. Thus, I would not personally say either is wrong. People often change the word to change their tone. This is the reality of the language, which examiners tend to deny.