Not funny anymore

We started introducing the potty to our 2 year old when he was 18mo and was showing extreme interest in using it.He was almost fully trained during the day with in about 10days. Now nearly 6 months later he has decided to ignore the potty altogether and pee and poo outside where our 10mo puppy does his business.
We found this to be cute and amusing the first time, but now as soon as he is put into a nappy or undies he is straight outside and naked within minutes going to the toilet as the dog looks on.
He is showing almost no interest in the potty and will not do anything if placed on it through out the day.

Does anyone know where I can start to break this habit?? We have a new baby due in a few months and I am desperate to have this sorted out by then.

He is copying his good friend, your puppy. I would suggest getting the puppy away from him for a little bit while you get him reacquainted with the real toilet routine. Start over, what worked before will work again. Then to strengthen that difference between people and dogs make sure the dog goes on walks for a few days to do its business when the dog comes back home. Additionally, If your son is outside a lot before he even needs to go pee, get a lemon tree and have him pee on that. It is great for the plant and having his own special place to pee outside will make him feel like a big kid. If he poops again outside after you have gotten him back on track, grab a big wad of paper towels and make him help you clean it up. No one wants to touch their own poo, even a two year old boy. (all from experience)