A relapse is new symptoms OR a resurgence of old symptoms (i.e. ones that had got better) that persists for more than 24 hours, and isn’t accounted for by any other cause, such as an infection.
Many people - probably the majority, I think - do have ongoing symptoms between relapses, because the damage from relapses is often repaired imperfectly, which means function is not 100% as it was before. Over time, this damage mounts up, which means people can accumulate more and more symptoms and disability, even when they’re not in an active relapse.
I’m not sure trying to persuade your OH to go to the gym more is the right tack, to be honest. I’m not saying the gym is a bad thing, and it’s good they’re going at all. But it’s very much down to the individual to manage their own levels of fatigue, and only they can know that, and what activity levels they’re comfortable with. Trying to persuade someone who suffers badly from fatigue to take on additional activity may be counter-productive, as there is a fine line between a beneficial amount of exercise, and overdoing it to the point of feeling worse. This line won’t be the same for everyone with MS, and your OH will know better than you where theirs is.
So if they feel they’re doing enough gym already, they probably are. It’s important, with MS, to listen to what your body tells you about how much is too much, and to respect that. Anything that feels alright is OK. If it doesn’t feel alright, and leaves you having to rest up for two days, that was too much. Unfortunately, sometimes you only find out too late that something was too much, but with experience, you do get better at judging. Still doesn’t stop the odd mistake though.