First, sorry for the long post. I'll try to give you some pointers that might help.
Your friend is probably in search of something. Escape from Armageddon seems to be part of it, as you mentioned. There could also be other factors motivating him. I would suggest focussing on your friend's motives more than on scripture per se. Except where scripture is one of his sources of fear regarding Armageddon, that is.
You could point out that other denominations have vaguely similar beliefs about end of the world scenarios, only they think you have to join their religion to escape doom and live forever instead of the Society's religion. Give your friend examples of these beliefs, go in to some detail and make them sound as plausible as the Society's viewpoint if you can. This might dull his fear of the Society's particular doomsday scenario. After all, your friend can only escape ONE of these doomsday scenarios. If you make them all seem plausible, then he may begin to realize that fear isn't a good enough reason to subscribe to the Watchtower's view above everyone elses. What if the Mormons are right? What if the Catholics are right? He can't escape every version of Armageddon. So why focus on escaping the Society's version? All of them conveniently want you to join them if you want to live forever, not go to Hell, etc. Fear just isn't the way to go.
With regard to scriptures that are making your friend worried about Armageddon, note that no Bible specifically say that the WTS is the true religion or say that you better stick with the WTS if you want to live forever. In fact, the Watchtower Society isn't even mentioned in any of them. The scriptures are actually quite vague on which denomination has "the truth." It could be any of them (or even none, in my opinion). There's no way to be 100% sure. So he can't be sure that God wants him to join the Society or that doing so will save him. That might actually make God angry and backfire. It could even be the final straw that guarantees your friend won't survive! Truth is, he just can't be sure.
This will hopefully help your friend realize that he is making several assumptions here: 1) that the Society must be the true religion 2) there is only one correct way to interpret scripture about Armageddon and that's the Society's way 3) that he truly understands what he needs to do to escape Armageddon.
In other words, his fear of the WTS's Armageddon could be totally misplaced. If it's true, he doesn't have enough information to save himself from it. He just has to hope for the best. He certainly doesn't need to go crawling back to an organization he clearly doesn't like. Why not try something that feels/seems right to him instead? There are surely other paths to salvation he would prefer.
If your friend cites the Society's attempts to prove they are the true religion to refute you, remind him that these arguments have serious problems and are far from irrefutable themselves. They simply don't prove that the Society has "the truth" per se and are therefore worthless. I have written an article here about this very thing that might help you. Bear in mind that this link goes to an atheist web site, but the article still applies to the case you're trying to make to your friend.
Again, sorry for the long post. Hope some of these ideas are useful to you.
IsaacJ