I used to care for a three year old who would put a snorkel on it and when it got hard attempt to parade around the house with the snorkel on, you are not alone and he is not to early.
Does he share a bedroom with his brothers? If not you could extend the talk to include the bedroom. Also extend the private areas to include any region covered by a bathing suit. It might also be the right time to talk about hygien if anything is going to come out of the body or go into his body besides food, that should be done in a bedroom or bathroom.
Right now you don't have to include what will happen in 7 years but, you can lay the foundation by talking about things like flossing, brushing his teeth, rinsing his mouth and spitting in the sink. garggling, blowing his nose, picking his nose, throwing up, going to the bathroom and if he has had to use suppositories you colud talk about that too.
It is important to cover that aspect because nature has away of surpising us before we have the chance to have that talk. My friends son had his first at night discharge at 10, he hid the clothes for a year in between his matresses until after the school had the "your body is changing" class and he found out what had really happened and told his dad.
You will do great,
L.