My SO wants to lose weight but I don't want them to.
Why not? It's for his health.
His weight is what make him look like manly
This is for his health. You wanting him to remain overweight is selfish.
Well I guess I should say it out. I am lazy and I don't want to date him if he is on diet.
You need to let him know about this as soon as possible, because he should find out that you aren't very supportive sooner rather than later. It's best if you both find new people if a simple diet is enough to get you to consider leaving.