You do not.
Wait for him to show interest first.
Oral sex it more or less about masturbating a male dog with the added benefit that in the end you do the masturbation with your mouth. So lets begin with how to masturbate a dog. Castration/neutering: The purpose of castration is the removal of sexual behavior. "Another benefit of castration is...
www.zoovilleforum.net
1. "Adolescence in dogs marks their sexual maturity, usually between 8 and 12 months."
(
https://pmcofedmond.com/pet-education/puppy-dog-vet/dog-stages-of-life/)
2. "When Do Puppies Reach Sexual Maturity?
The timing of sexual maturity is largely size- and breed-dependent. So, talk to your vet and breeder about your puppy. Sexual maturity in puppies typically
begins to occur between 6-to-9 months of age, although certain
giant breeds may not reach sexual maturity until they are older."
(
https://www.akc.org/expert-advice/health/sexual-maturity-in-puppies-what-to-expect/)
3. "Sexual Maturity. The onset of sexual maturity (puberty) is triggered by hormonal changes that lead to physical and behavioral changes. Generally speaking, puppies will
start to mature sexually between 7 and 10 months of age."
(
https://www.guidedog.org/PuppyRaising/PuppyRaiserManual/CareAndHealth/Sexual_Maturity.aspx)
Generally speaking you should not attempt to masturbate a dog until he is sexually mature. You should wait for the dog to show sexual behavior like interest in female dogs and mounting/humping.