He'll manage just fine.
He should be able to cover her. I would have an experienced vet or breeder with the horse in case you need to help him manually.
If you have to ask questions you shouldn't be breeding any thing.
Judy and the Beast
If the stallion is old or arthritic he may have trouble. I had a 14.2 mare covered by a 13.2 stud and he needed to stand on the top of a hill with her on the lower side to get the job done.
Very difficult but it can be done, They will need to dig a hole for her to stand in or have a ramp for him.
Hah hah a 10 hand stud can climb on her if she lets him!
Let the stallion decide. If he is hesitant, you might have to put him up to it. ;-) It's sort of like the astronaut who landed on a planet and was met by a gorgeous woman who was nine feet tall. His first words were, "Take me to your ladder; I'll see your leader later."
No body allows there stallions to live cover any more.