What is the life expectancy of stage 7?

That certainly is an accelerated decline. Unfortunately there is no predictable time frame for any stage. Did he experience a traumatic event? A surgery, for instance. That could cause a rapid decline. Normally the disease progresses over a 8 to 20 year time. What indications have you noticed that makes you believe he is in stage 7? What specific symptoms do you see? Keep in mind that dementia presents itself not so much in stages but as a continuum. No single behavior defines what stage a person may be in. He may show a stage 7 symptom but may actually be in 6 or even 5. You have to look at the big picture. But even 5 or 6 would be a rapid progression from 2.

There is no life expectancy to any measurable extent. You can be given averages such as "given a diagnosis of _________the expectancy is 5-8 years". Over all that isn't very helpful. Moreover Docs are often hesitant to give you "expectancy" after having been "held to them" by distraught survivors ("But you SAID she had 4 More YEARS" for instance). As my cancer doctor said when I asked if I should take a certain medication or not "Oh.I don't know. It's anything but an exact science" and then smiled and said "I can only say that to you because you're a nurse. People want answers. So I give them answers".
Much also depends upon condition over all, and then on the amount of exercise and good living the person is able to maintain. It has been found that even in macular degeneration now, a sedentary lifestyle brings it on earlier. So there are so many things that figure in. There is EVEN the fact that a person requiring say a general anesthesia for a procedure can deteriorate very rapidly.
The best one to give you SOME idea of your own question regarding a particular individual is likely the doctor following that person.
Best of luck; hope you get some sort of answer.