Are you home a lot to be able to work with him. It sounds like you probably have a fence and you let him out on his own. If that is the case I would start using a long leash or tie out and go out with him each time he goes out to potty, I am not sure what type of bell you use, but mine is on a ribbon with some small jingle bells, you can gently ring it before going out and give him a small treat right by the bell, then go out but don't walk all over the place, because he is out Just to go potty not to play and chase things, I find the less Lola can walk around the quicker she will go. If I let her pull me all over the yard she would sniff for an hour, so stand your ground. After he goes, then go right back inside with a small treat. Go out ever hour if you can and when and if he poops outside make a big deal and of course treats. Not bothering to do their business outside and playing instead is common. My sister's yorkie has a big yard and it will be all over the place and she can not see if and what she does. Then later on she has accidents in the house, because when she went out she played instead of going. Lola goes out on a lead and 90% of the time she tries to get right back in after peeing, when I know she has not gone poop in quite a while, or in the morning, I will stand on the porch blocking the door and say go poop, go poop, go poop, it's funny she will look at me, put her tail down and then go back down the steps and about 90% of the time will poop. After about 5 min and no poop then I figure she must not have to go so we come back in. It's a pain to do this but I found it very effective. My husband often does not want to take the time, so I go through this with him, LOL not trying to make him go, but proving to him when he takes her out just because she did not go does not mean she does not have to go. He will say she does not have to go, I will say yes she does, then I tell her go potty, go potty, go potty and low and behold she goes
LOL when it comes to my DH I love being right. My neighbor has a large dog, sometimes I will let the dog out for her if she has to work late, even though her dog has been in the house for about 7 hours, it goes out and just stands there and would come back in if I let it. I have no idea why so many do that, it's almost like they are to lazy to go. So that's why I think having them roam loose is not always the best idea for some dogs. Also before we go on a walk I do the same thing I walk around the yard with the potty lead to see if she will go, then I switch to our walking leash, this way we are not half way into our walk when she goes and then I have to carry a stinky bag around with me. The last thing I do before heading out for a walk is to use the bathroom, so think the dog should do the same. Anyway good luck I know some dogs are easier than others.