He isnt doing it on purpose he probally just doesnt know exactly what he is going outside for, when you do take him out are you giving him a SUPER GREAT treat and a huge GOOOOOOOOOOOD BOYYYYYYYYYYYYY,??? I used boiled chicken when she would go outside. Everytime she went I would say HURRY HURRY. She liked the chicken and the praise so much she would often squat and fake it just to get the chicken and the wooo hooo's!!! Give him time he is still really young it takes a good year to potty train, may I suggest a Belly Band which you can put on him right when he walks in the door to the house, this way there would be no accidents or markings.
FuFu Fashions Belly Bands, Doggie diapers, Dog Diapers
There are also others who make these on this site, and they do prevent them from peeing in the house !!