Michaela,
I agree with Dawn. Diet is probably the key. If it isn't goopy all the time it may be related to specific foods. I know that I had a rabbit that got goopy poo everytime I gave him carrots or those kaytee yogurt dips for rabbits. I just had to quit feeding him those things.
My rabbits get plenty of timothy hay (always available to them), water and good quality pellet in the morning and evening. I also feed them fresh veggies in the morning and evening but stick mostly to leafy greens (cilantro and green lettuces are their favorites). Sometimes I give them small quantities of canned pumpkin or fruits but these are more like treats so they are limited. If it doesn't seem related to a particular food or treat then I'm not sure.
There are others on the forum more experienced than I with this so I would keep posting in this section and try to give as much information about what your rabbit is eating. He's still pretty young, isn't he? Is he eating alfalfa hay still or timothy? I've read that with young rabbits you should introduce veggies slowly and one at a time.
I hope that helps a bit.
Kirstin