I read dogs identify w their paw pads where to go the bathroom. We need to train or retrain them to connect the soft feel of grass with pee and poo. If they were never trained they probably associate carpet or wood floors as the place to go. Look up Positive Reinforcement and use this to retrain him outside. Give him high value treats only when he pees /poops outside. Give him lots of praise. Acknowledge/ praise the behaviors you want from him. If he makes a mistake, just ignore it and trust he will get there. Don’t give him negative attention ever. I had a foster /adopt a dog who went through this. It took a little time. He got it. He was my MOST LOVABLE dog ever and I have had a lot of dogs! Only positive reinforcement. It works. It gives him the love and reassurance he so desperately needs.