Hi, I'm Dr Steve. I am assuming that you are talking about having him gelded (castrated). Is that correct? I will assume again that is what you are concerned about.
I typically counsel clients that gelding can be done at any time after they have approached sexual maturity. This can be at or around 1 1/2 to 2 years old. The benefit of waiting until this time is that the horse will more fully reach his mature size. In some horses, gelding can slow their growth, so if a horse is gelded too young he may not get as big as he would have if not gelded early. The amount of the growth affect usually is not more than 10 % so it is not a huge difference, but the growth can be affected.
The other consideration is how the stallion behaves. Some stallions start acting aggressively and become hard to handle before they reach this age. In order to help make a horse more trainable and reduce the chance of an injury to a person, I recommend gelding at the time of the horse becoming difficult to handle . If a horse doesn't become difficult to handle before the age of 2 years, then gelding at 2 years is another good time. By this time they have either reached their full mature size or are very close so gelding at this time is not a problem.
As for cost, that varies significantly depending on the area of the country you are in. I am a mixed animal vet so I have a general practice ,not an equine specialty practice. I charge a total of about $150-$175 which includes my trip charge, anesthetic, gelding ,antibiotics pain medicine and a tetanus vaccine. THis may cost more if you have the procedure done at a specialty practice.
A more accurate way of determining how much it costs would be to call several practices in your area and get prices from them. Then ask for references of clients they have gelded horses for and then you can make an informed decision. Remember the cheapest may not be the best so see what their clients have to say about the vet.
I hope this has helped. If it has would you please rate my answer. Thanks for the question. I will check later this evening to see if you have more questions. HAve anice evening.