You do not have to make any decisions at the his moment in time, but you should definitely get proper legal advice asap!
You will be better placed to make those decisions when you are fully informed of all the pros / cons and possibilities of your particular situation.
You H has basically been living a polyamorous lifestyle for almost a decade, with at the the emotional closeness and support of two women, although (hopefully…) only the physical closeness of one???
As he sits there ‘sorting his head’, trying to work out how he loves her / you etc, there is unfortunately nothing to say he won’t take the decision out of your hands. He could come back and ask for some sort of separation scenario to sort his head or for more permanent space or some other random ‘reason’ to split.
You (and apparently he) have no idea where he is at wrt the OW. Having had en emotional relationship with her for 8 years he views it as totally normal for you all and will likely not understand why you have a problem with it now.
Basically knowledge is power. Get informed asap. You don’t have to act on the information, but you need to know what it is in order to make the best long term decisions for your future.
Ps don’t get too hung up on the loan issue - don’t let it dictate your future. You need to choose your future and then sort the rest. Yes there will inevitably be some changes- but those will be what you need / choose.