He's only eight months, is this okay?

I have a little puppy his name is bruce, he's 8 months and hes fixed if that matters. Anyway I take him out ever morning around 8 and throughout the day. Sometimes he goes, sometimes he doesnt. Most of the time he doesnt. I'll have him out there for like 30 minutes and then he comes in the house and goes. I have four other dogs and none of them go in the house. We have a doggy door so they can go out whenever they want. I dont understand why he always wants to go in the house. How do I stop him?