Can you cut the grass a little shorter for him? As he gets bigger you'll just have to deal with wet feet. Sammy is really short but she still does fine in the grass. He is not going to be housetrained in 1 night. It will take several months. Make sure you bring treats outside with you and make a big deal about him going in the grass. He needs to learn why he is in the grass in the first place
Don't try to confuse him with puppy litter and pee pads and everything else. Just reward him for going in the grass and keep an eye on him when you are in the house! You are doing great if you already caught him pooping and took him to the right spot, wherever you decide that is. You'll need to crate him when you can't watch him, like when you are in the shower or cooking dinner. Expect accidents until his body can learn to hold it (which he can't do right now).
Good luck!