He is probably stressed out so understand that he will not be behaving his best right now, he is in a new and scary place and doesn't know what will happen next. He will continue to be scared and annoying and difficult until he gets settled especially if you are taking him somewhere else tomorrow. Resist the urge to tell him off loudly or yell, he will have no idea why you are being so mean and punishing and scaring him. If he does it inside, keep an eye on him so you can straight away run over there and take him outside, saying firmly and loudly and slowly (but not yelling) 'NO.' No exclamation point, just 'NO boy DON"T do that'. Take him out mid-stream if you must (hmmm might want to keep him near the door or on laundry/lino type floor). When he wees outside tell him immediately 'Good boy! good boy!' and give him lots of affection. And I second the idea to put the pissy rags/whatever out there with him to get the idea. He might also be encouraged to go on the same trees your other dog has if you take him over to those trees right after the other dog has finished.
Remember he just doesn't realise that he can't go inside, don't get frustrated, just make it clear that you do not like him peeing inside and you disapprove and peeing is for outside only. It might be out of excitement or nerves inside, or he has lived inside and weed there so he doesn't realise that's not the rules in a new place.