However, it should not be overlooked that he is still very much a living breathing animal, rather than a cuddly toy. As such it requires time and effort to train him to toilet in the right place (outdoors.) To assume that because he looks cute he has cute habits is setting yourself up to fail. Just as much work is required to housebreak a pug as a Rottweiler or Doberman.
Right from the beginning, start as you mean to go on. Show the pug where the toilet spot is, stay with him, and praise him when he goes. This is balanced against keeping him supervised while indoors so that he doesn't get a chance to get into bad habits.
Until he reaches this happy state you need to put plenty of effort into making 'the penny drop'. This includes giving him ample opportunity to go outside, but this must always be under supervision. It's an essential part of training that you are with him so that you can praise that magical moment when he urinates outside.
If you merely put him outside to leave him to his own devices for half an hour, the pug isn't going to realize this is his chance to toilet and at best won't learn anything but at worst he'll play for 30 minutes and then return inside to pee... much to everyone's frustration.
The important thing is to apply the rules consistently and to keep going... you will get there in the end if you keep plugging away.
To teach a pug to pee outside you need: