What are you feeding him? Maybe it's a food which is making him have to go more than others? How much does he go?
The only way to stop him from going inside is to stop him from going inside. LOL. (isn't that a great help?) Really though...the more mistakes you make where you're not supervising him enough and he goes, that reinforces this behavior. It's a payoff for him to just go inside. It's easier. He doesn't have to wait, get cold, go away from where he's comfortable...it's convenient to go inside. The carpet is absorbant...dog's like a place which absorbs the scent. That way it stays there for all other dogs out there who will know he was there. So, if you can't supervise him, you'll have to confine him to a crate (I hope he's used to a crate) And you'll have to take him out more often.
You can also start writing down when he eats, drinks, naps, plays, goes pee and poo. That way, you'll have a little something to go by as far as his schedule and be more in the know when he'll have to go. (Hey, I'm a poet)
It's hard when they're older like that, but it can be done. You'll just have to be extra extra diligent and prevent accidents. You're doing great rewarding him when he goes outside. I wouldn't scold him at all inside because that doesn't work too well with potty training. He can think you mean that he shouldn't go at all, not that he shouldn't go inside. Outside, give a command word while he's going (keep it low key) and the the second he's finished, praise him a lot...probably what you're already doing.
My little girl Chi had that dirty puppy syndrome and never learned as a tiny pup to go outside. She's been more of a challenge. I have to let her out, probably more often than necessary, just to make sure she gets a chance. She will let me know, but it's so subtle that I can miss it sometimes and then she won't hold it very long and will go if I don't pay attention or if I'm not here to let her out. My boys will wait as long as they can. She won't try as hard to hold on.
Don't give up.