If the possum is truly alone and only 5-6 inches in body size, it’s probably an orphan. I’d suggest getting in touch with a local wildlife rehabilitator since at that size, they’re usually not self-sufficient yet. However, mother possums sometimes “plant” their older young in a safe location while they’re out foraging, and if there’s any chance there’s an adult around too (he should check the shed during the day to see if the mother is there), then I’d just leave the baby alone since it won’t be much time until he disperses.
But if the possum’s body size (nose to base of tail) is actually 7+ inches, he IS self-sufficient and so I’d suggest waiting until after dusk — when the possum is likely to leave the shed for his nightly forays — and then sealing up its entry hole so he can’t return. It’s important to underscore that possums are totally harmless and rarely carry rabies, so doing nothing is often the best and easiest solution — possums are generally nomadic and will move on on their own anyway. Also be sure there’s no catfood/dogfood left outside in the vicinity which is encouraging this possum to stick around!
By the way, there are no repellents that will keep an opossum away.
– Hire a nuisance wildlife control operator or contact your local animal control. They can capture the animal and release it outside (after you close the door). This is best done near dusk so the animal is most likely to move on with darkness approaching.
– Set a trap. Preferably use a 10x12x32 inch size single door trap. Bait with something fatty, like chicken skin, tuna in oil, sardines in oil. Close garage door. He should be in there the next morning. Then release. Details on trapping and trapping safety can be found at ICWDM.