Help! small-cell cancer treated only to find Liver & Adrenal Cancer via PET scan!

Help! I am at a loss for words! My ex-husband just went through agressive chemo & radiation for his small cell lung cancer (also in 2 lymph nodes in his neck). The final xrays showed it was miraculously GONE! They did brain radiation as a, just two or three months later,(still feeling nausea & sick all the time) they finally gave him the PET scan he had been wanting...came home today to call our children to say They found the cancer via the PET scan, in his Liver & Adrenal Glands now!!! They said 2 months without treatment and maybe a couple more With treatment! HOW ACCURATE is this and how do they treat it AND could it have been found earlier if they had ONLY done the PET scan earlier, when he was first diagnosed??? (HIS Dr.'s say it wouldn't have changed anything)! I disagree! I am angry and scared and heartbroken...especially for our 3 adult children AND my ex!! He is only 46 yrs old! Why would they NOT do the PET right away?? What are the options, outlook, what is to come? Any advice or info on any of the cancers or tests and treatment...anything, would help me feel less helpless and angry and totally devastated!! Thank you.