Toilet training a Rescue Dog

We've had Bosley sine Monday evening. He's spent the last 3 months in the Rescue Centre, and the first 8 months of his young life are unknown. He must have clocked up over 3 hours of walks, mostly in rural areas. However, despite letting him sniff to his heart's content, Bosley hasn't done any 1's or 2's. He's done a few in the back yard, and has been praised with pats and treats. Equally, he's made a few house messes, and I recognise the problem relates to his Pen life of "soiling his pad".
I'd appreciate any advice on getting through this problem.

Yep a common issue. When he makes a mess in the home ignore it, no fuss no telling off just clean it up with a deodorising cleaner.

Now, when he goes outside in the garden give loads of praise and treat. The same should be done when out on walks too. This may take some months but I assure you it works and you will get there.

Now there is of course the typical Staffy issue of finding the right place to go when out for a walk... OMG it can take an age for them to decide the right spot

As well as Ian's advice (spot on), I'd go back to basics as you would a puppy. Let him out in the yard very frequently and make sure he goes out first thing, after every meal and last thing. See if there is otherwise a pattern to when he goes and, if there is, anticipate his need. Then, responding as Ian suggests, you should give him every opportunity to get it right, which you can then praise and reward.

Echo Ian and Liz's advise,lots of praise and patience

Thanks for the replies. Yes, I'm acting normal with the house messes. No noise,no reprimanding and no negativity. It will take awhile, but the testing moments are far outweighed by those priceless good moments that are already coming in abundance.

They are so worth all the early testing times

Rescues do indeed give back so much more than they ever ask.

It sounds like Bosley's not had much training so assume that he knows nothing and approach his training as if he was a puppy. Slowly but surely, you'll get there.

Lovely, lovely looking little fella, well done so far. Wanted to say you might want to keep an eye-out if you've been playing for a while. Gizmo used to get caught up in the game and not realise he needed to go until it was too late.