. Thank you for mentioning the fear stages, let's hope it goes by quickly! 
Sudden behavior change at this age is almost always medical. I am glad you are now keeping him separated, but honestly you should have the first incident. After the first bite, most courts or judges would rule you knew he was a danger and he could not only have been seized, but you sued for significant damages. You are lucky no one is filing.
Thyroid is the primary cause that I am aware of for sudden change. Yes, brain tumors are possible, but more likely thyroid. Some forms of epilepsy also can cause a dog to attack, but typically this is truly out of nowhere-- not like what you describe when a dog is told no about something. Pain can also do it.. sadly cancer had been found in similar situations of sudden growing aggression. The ENCOURAGING part is you had several months without problems... so again, look at the most simple... thyroid. Make sure you get a full panel, not just an in-house partial.
In the meantime, understand you didn't cause this, and it may be fixable. If thyroid, it's inexpensive medication. But you won't know until you get tests runs. Hopefully those wanting him put down will feel better knowing you are acting to find out if there is a medical issue.
