Are you doing strict outdoor training? Or pad too? Is he neutered?
A shirt like this or a belly band is not going to help potty train. It will prevent accidents, but it won't really potty train. You do run the risk of him learning just to pee in the shirt, and then you have not solved your problem.
My boys have a piddle pad available to them 24 hours a day at home. It is best for our lifestyle and enable them to go frequently.
Great advice above from Paisleypixels. The keys to success are limiting freedom while training, supervision whenever possible, and loads of praise/reward when he gets it right.
I have a post about potty training on my blog:
Potty Training Small Dogs - Dog Life and Style
Good luck!