
I would say it's purely at your discretion. Return stud arrangements to the same dog are by agreement anyway, not inherent in the paying of a stud fee, and there may be various reasons why your other dog is not suitable. Also, as he produced a large litter soon after mating their bitch, there should be no questions on his fertility.
However, that said, assuming he's already been used at stud and there are no specific reasons not to use him on that bitch, I'd be inclined to take the easy option myself and keep the peace - always nice to see what a young dog will produce to different bitches anyway. He will not care that he's young and you'd like to take it slowly with him, he'll be very keen to oblige I'm sure :D
Marina