Hi there and thanks for the question.
Firstly he is still young so bladder control can be a bit much for him.
I would encourage you to take him to the vet so we can be sure there is not an underlying bladder reason for his mistakes.
If there is no bladder problem then i think a combination of time and the below method will help the situation.
The next time your dog has an accident inside:
1) Growl as your dog is doing it. You must reprimand your dog as it is performing the undesirable behavior, because dogs only ever associate your punishment or reward with the very last action they have performed.
2) Soak up the puddle or pick up the waste with a sheet of newspaper.
3) Then clean the spot with your odor neutralizer. I like SOX as a product but ask you vet what they think would be best.
4) Keep the soiled newspaper, place any solid wastes or the soaked paper outside, or wherever you want their bathroom spot to be. This will give your dog a place it can return to that is marked with its own scent and therefore safe to use as a bathroom. (keep in mind that dogs mark their territory and therefore claim their dominance as they eliminate, if another dog has been where you want your dog to go, your dog will only go there if it is of a dominant disposition and wish to challenge the opposing 'dog'.)
5) Take your dog out every hour on the hour (when possible) to that place where you have placed its waste. Introduce the command "Be quick!....Good Dog" as your dog goes and reward it with treats if you wish. This command is handy, as your dog quickly associates your voice command with the act of eliminating. My dog's will even try and go anytime, anywhere if I give this command, knowing the treats that usually accompany being obedient! (even if they have no juice, I still reward them for their efforts!)
Make sure you only tell him off when you catch him....not later.
Good luck and please let me know how things go
Todd