I just wanted to second the crate training recommendation. We crate trained our basset hound and it worked wonderfully. Just be sure not to make the crate a punishment thing. It should be a place that they feel comfortable and safe - not a place that they hate. Practice letting him walk in the crate, give him a treat, in and out like that until he gets comfortable getting in there. Crate him overnight and while you're out for the day and be consistent. Also, be sure to give a LOT of praise for going to the bathroom outside - not only treats but also petting and plenty of love. Lastly, be patient... it took our dog 6 months to be potty trained. She was full grown when we got her and had been abused. It can take at least 6 months to potty train a full grown dog.
If these things don't work, I'd advise taking him to the vet to rule out any medical issues such as a bladder/kidney infection.
Good luck!