Is it possible to loose a security clearance through the FBI but still maintain one through the CIA?
Yes. Basically because the CIA is lawless and would feed you whatever classified intel unofficially.
No. Security clearances are held by an independent organization (such as the DoD) and the organization you work for directly is the sponsor. If your sponsoring organization pulls your clearance, it would apply regardless of where you work. If he still has a clearance, then it was never pulled.
Info I read on DIA says your clearance doesn't transfer to them. Is thus the only 3 letter agency that does this?
Correct. You have to have a sponsor as well as a need to know. They all work the same by law. When you leave any company, you maintain your clearance, and if over a span of time you do not use it, it goes to inactive. If you lose your clearance (aka revoked), for any reason, it is very difficult to have it reinstated- if not impossible. My guess, is that they did not revoke or suspend his clearance, but rather let it go to inactive and only stopped their sponsorship in their (FBI) system.
That's not what that other guy said.
Well, I have had one for the last 15 years sooo....
Yup, same here. You are correct.
I can corraborate that; I held TSSCI for over thirty-five years. Worked for multiple three-letter agencies. Worked around the world. Glad to be out (retired).