I just wanted to say "Thanks" for rescuing Iggy! I have a rescued chi myself and we just adore her beyond belief.
The above advice is good, but you might consider crate training as well. The advantage to that, if you can be home a LOT, is that when you are not actively watching him, he will be in a crate and not going on whatever is left on the floor. It's a bit tricky since he is older and possibly would hate the crate...you would want one that is cozy for him with quilts and toys in it, (we have ours in a secluded part of the house...it's nice for napping). I crate-trained Dolly and she was extremely successful with it.
I wouldn't want him to be upset though, so it's your judgement call. The good thing is that you have time for him and wouldn't have to put him in there for hours.
Dogs will NOT go in their crate, unless they have room to make a "potty" in a corner of it, so make sure it's not too big. The routine is to have him sleep in there at night, then take him straight from the crate to outside. He will HAVE to potty, so you will be sure of "catching him being good". You can praise and dance and even give a bit of cheese (have it handy, because early morning is a great time to know he'll perform for you). Then bring him in, feed him, snuggle and if you can't watch him (say, you're doing dishes, or whatnot), put him in the crate again. After about an hour, take him out and back outside. Since he will have eaten, he'll have to go again, and once more, you can catch him being good. After a couple of weeks, if you want, he certainly wouldn't have to sleep in it, but don't let him have free rein in the house...Dolly now sleeps in our beds (with any of us!), but she doesn't get off the bed in the middle of the night.
If he hates the crate, you can also try keeping him on a leash, with you at all times. They make pretty good signs that they have to go (whine, circle, etc), and you won't miss out on them if you have him leashed to you...then go out, praise him for being good, and go back in for the next 2 hours or so.
They usually have to go after eating, sleeping and playing...and if you go out the same door every time, he will get the idea and go and wait there. Dolly now goes to the door and gives us a funny little bark, and we take her out. She was 4 months old when we got her, and completely untrained, and within two weeks, she was housebroken. He will probably catch on quicker, because he can hold it longer than a pup.
Sorry this is so long, but I'm a big advocate of crate-training. Maybe it will be what works for you. Also, just having a crate is nice if you have to go somewhere and you want to know he is SAFE. It's like with kids...they don't like their carseats, but we put them in them anyway, because they are safe.