Neutered male marking in house? PLEASE HELP?

My neutered male GSD has started marking inside. I've tried starting toilet training again (eg. Crating when I can't watch him.) but when I let him out he'll mark infront of me! I've tried sprays to deter him with no luck. I've purchased a belly band so he can't urinate on my furniture but, it's not something I want to have to keep on him constantly. I've also taken him to the vet, and they ruled out any medical related issue. PLEASE HELP! What can I do?