Be mindful of the fact that just because he is a US citizen, a 6 mo visitor pass is not guaranteed. It's totally dependent on the border agent handling his entry at the time, and if that agent feels he is misrepresenting himself in some way, he will be denied entry or given a pass for a specific date he must leave by. It's important that he convinces them that he is only going to VISIT you, as going to Canada for the intention to become common law is not a reason to be allowed entry. It is also important that he provides proof of his ties to the US when going to visit. Since he is retired, this can be a lease agreement or mortgage papers, and possibly his SS benefits. They do not want to see him bringing all of his belongings only to visit. Especially because you are not married, it will be more suspicious. Also remember they have every right to go through everything - computers/phones/emails and will use information they find against him if they deem it as proof of misrepresentation. Basically, if he has emails, documents or texts about "moving" to canada or shipping belongings, then they need to be erased or hidden before he goes to the border. I would think his age and retirement helps him in being able to leave the US for 6 mo. They want to know he has something to go back to and that his intention isn't to live there illegally. A month before his pass expires, you can apply for an extension. I am not sure what the approval rate is for people without PR applications in process. They seem to give leeway to US citizens with applications in process. Again, just telling them you want to become common law isn't a good enough reason to be given the extension. They want you to follow their rules and do it their way, not make up your own!
In order to be considered common law, you must be provide proof of living together for 12 months. This includes joint bank accounts, bills in both names, a lease with both names, a mortgage with both names, etc. It's suggested a bit more research is done on these threads about common law applications, as it requires a lot more proof than spousal. My guess, if he wants to stay with you during the whole time, it's best to apply inland. However if you apply inland, then he will not be able to leave canada during the application process or else it will be considered abandoned. If he applies outland, he can go back and forth frequently while the application is in process, and request extentions to stay.
In regards to losing SS or pension, you would need to contact SS and the holder of the pension directly and ask them. There may be rules about living abroad and being a PR of a foreign country in order to collect. My guess is since being a PR does not mean you are giving up US citizenship, and he's paid into the system already, he can still collect. Again, I don't work for SS, and am not collecting it, so i'm not familiar with the rules.
Keep in mind the inland app process takes well over a year, and outland takes an average of 8-10 months. I believe in order to stay in good status as a PR you two have to be living together continuously for 2 years. I am not sure what that means in regards to moving to Costa Rica. I am sure they have their own rules about living there long term.
Again, there are a lot of threads on here that can be helpful to you, just keep scrolling and reading! best of luck!