OK I'll have a try.
A male dog can be sexually mature when he is about five months old, but this does not mean he is physically or mentally mature. Even if presented with an in season bitch he may not be interested. Thats the physical side.
Now for the moral side and also the practical side.
Unless your dog is an extremely good specimen of the breed, sound in mind and body, excellent temperament,
healthy (no eye, heart, lameness problems), from parents that have the same qualities (do you know the parents?) very few people would be interested in using him at stud. Most bitch owners would select an experienced stud dog, owned by an experienced dog owner. Even then unless the dog is exceptional they are unlikely to want to pay very much for his services. Breeders often have their own dog, so would not want to pay for someone elses. Usually the first service is given free anyway, to 'prove' the dog as he may not be fertile or may throw very small litters.
Once your dog has mated a bitch his temperament may well change. He may start lifting his leg indoors, mounting your leg or any young children in the house. He may try to escape if he can smell a bitch, or howl the place down as long as there is an in season bitch around. He may not be interested in spending time with you or your family, or in playing normal dog games.
If he is only used on the odd occasion, is it fair to subject him to something he has no control over, or to subject yourself to constant grief for the sake of the odd (small) stud fee?
Do yourself a favour - get him castrated and leave it to the experts.