I know it's good for people with depression to get out of the house, so how could I motivate my Fiance to leave his house?

You could motivate him by telling him that going outside will improve his mood and make him feel a bit happier. You can also tell him that going outside is refreshing and will help him with his energy levels. You can also mention health benefits if necessary. You could also go outside with your Fiance for his own comfort. If your Fiance likes to spend a lot of time with you, this could also be used for motivation. Try to give him options on what could be done outside of the house, like going to a park or a restaurant or even a cinema.

Try suggesting doing something they enjoy and offering to engage in it with them. Sometimes even offering to go somewhere they know would put you in a silly situation might make them interested. For instance, I’m super awkward and clumsy so I’m sure if I suggested to my fiancé let’s go rock climbing (something they like but I suck at) would get their attention. Let them know you support them and love them. I would try to refrain from pushing it though. If they refuse don’t be discouraged just let them know you understand and suggest that maybe later you could if they feel up to it.

As someone who has dealt with depression myself, it can definitely be a struggle to not only get out of the house but as well as get out of bed. Sometimes small things can be a really big battle. It's good to start by encouraging them to do the little things, and support them when they do. Tell them that you are proud of them for getting up! Try asking about their favorite things to do or if you already know what they are, suggest a brief little "date" For example, if you know that your Fiance likes coffee, suggest taking them to their favorite coffee place and then come right back home and spend your day together! It's all about taking small tiny steps and celebrating them along the way